A Photorobust Mo(0) Complex Mimicking [Os(2,2′-bipyridine)3]2+and Its Application in Red-to-Blue Upconversion

Article abstract of DOI:10.1021/jacs.0c12805

Osmium(II) polypyridines are a well-known class of complexes with luminescent metal-to-ligand charge-transfer (MLCT) excited states that are currently experiencing a revival due to their application potential in organic photoredox catalysis, triplet-triplet annihilation upconversion, and phototherapy. At the same time, there is increased interest in the development of photoactive complexes made from Earth-abundant rather than precious metals. Against this background, we present a homoleptic Mo(0) complex with a new diisocyanide ligand exhibiting different bite angles and a greater extent of π-conjugation than previously reported related chelates. This new design leads to deep red emission, which is unprecedented for homoleptic arylisocyanide complexes of group 6 metals. With a 3MLCT lifetime of 56 ns, an emission band maximum at 720 nm, and a photoluminescence quantum yield of 1.5% in deaerated toluene at room temperature, the photophysical properties are reminiscent of the prototypical [Os(2,2′-bipyridine)3]2+ complex. Under 635 nm irradiation with a cw-laser, the new Mo(0) complex sensitizes triplet-triplet annihilation upconversion of 9,10-diphenylanthracene (DPA), resulting in delayed blue fluorescence with an anti-Stokes shift of 0.93 eV. The photorobustness of the Mo(0) complex and the upconversion quantum yield are high enough to generate a flux of upconverted light that can serve as a sufficiently potent irradiation source for a blue-light-driven photoisomerization reaction. These findings are relevant in the greater contexts of designing new luminophores and photosensitizers for use in red-light-driven photocatalysis, photochemical upconversion, light-harvesting, and phototherapy.

Full text of DOI:10.1021/jacs.0c12805

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A Photorobust Mo(0) Complex Mimicking [Os(2,2′-bipyridine)3]2+
and Its Application in Red-to-Blue Upconversion
Jakob B. Bilger, Christoph Kerzig, Christopher B. Larsen, and Oliver S. Wenger*
Cite This: J. Am. Chem. Soc. 2021, 143, 1651−1663
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ABSTRACT: Osmium(II) polypyridines are a well-known class of
complexes with luminescent metal-to-ligand charge-transfer (MLCT) excited
states that are currently experiencing a revival due to their application
potential in organic photoredox catalysis, triplet−triplet annihilation
upconversion, and phototherapy. At the same time, there is increased
interest in the development of photoactive complexes made from Earth-
abundant rather than precious metals. Against this background, we present a
homoleptic Mo(0) complex with a new diisocyanide ligand exhibiting
different bite angles and a greater extent of π-conjugation than previously
reported related chelates. This new design leads to deep red emission, which
is unprecedented for homoleptic arylisocyanide complexes of group 6 metals.
3
With a MLCT lifetime of 56 ns, an emission band maximum at 720 nm, and a photoluminescence quantum yield of 1.5% in
deaerated toluene at room temperature, the photophysical properties are reminiscent of the prototypical [Os(2,2′-bipyridine)3]2+
complex. Under 635 nm irradiation with a cw-laser, the new Mo(0) complex sensitizes triplet−triplet annihilation upconversion of
9,10-diphenylanthracene (DPA), resulting in delayed blue fluorescence with an anti-Stokes shift of 0.93 eV. The photorobustness of
the Mo(0) complex and the upconversion quantum yield are high enough to generate a flux of upconverted light that can serve as a
sufficiently potent irradiation source for a blue-light-driven photoisomerization reaction. These findings are relevant in the greater
contexts of designing new luminophores and photosensitizers for use in red-light-driven photocatalysis, photochemical upconversion,
light-harvesting, and phototherapy.
ligand-centered (LC),32,33 or metal-centered (MC) excited
INTRODUCTION
The development of new types of photoactive complexes from
Earth-abundant metals and the use of low-energy light to drive
■
states.34−40 Among MLCT luminophores, arylisocyanide
complexes of zerovalent group 6 metals represent a promising
class of compounds. Building on a few studies from the
1970s41,42 and newer work on emissive W(0) complexes with
monodentate arylisocyanides,43−45 we discovered that chelat-
ing diisocyanide ligands provide access to Cr(0) and Mo(0)
complexes (Figure 1A) as Earth-abundant analogues of the
[Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) lead compound.46,47
Our previous reports of luminescent Mo(0) and Cr(0)
complexes all employed chelating diisocyanide ligands based
on a m-terphenyl backbone. Of particular note, [Mo(L2)3]
exhibits strong MLCT absorptions out to 550 nm, and bright
3MLCT luminescence at ∼600 nm, substantially improved
with respect to [Mo(L1)3]which was assigned to the
sterically bulky tert-butyl groups protecting the metal from
the chemical environment.48
thermodynamically challenging photoreactions have become
two important interest areas at the interface of coordination
chemistry, spectroscopy, solar energy conversion, and synthetic
organic photochemistry. Our study addresses both of these
interest areas by disclosing a new design concept for Mo(0)
isocyanide complexes allowing the establishment of deep red
emission from a metal-to-ligand charge transfer (MLCT)
excited state, and its exploitation in triplet−triplet annihilation
upconversion to generate blue output from red input light.
Polypyridine complexes of precious d6 metals such as
Ru(II), Os(II), and Ir(III) have long played a dominant role as
luminophores and photosensitizers in inorganic photophysics
and photochemistry,1 and they continue to be essential for
organic photoredox catalysis2,3 and applications in organic
light-emitting diodes (OLEDs),4 light harvesting, solar energy
conversion,5−8 and phototherapy.9,10 Traditionally, there has
been much focus on Fe(II) and Cu(I) as Earth-abundant
alternatives,11−15 and some remarkable advances have been
made with these metals lately.16−26 In parallel, a broader
spectrum of base metals and oxidation states has come into
focus recently,27 leading to new types of complexes with
photoactive ligand-to-metal charge transfer (LMCT),28−31
Received: December 9, 2020
Published: January 12, 2021
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a deeper penetration depth into reaction media.54−56 Recently,
these aspects and the idea of using low-energy input light to
drive photoreactions that would normally require UV or blue
irradiation have gained increasing attention in photoredox
catalysis.55,57−68 The sequential absorption of two low-energy
photons per catalytic turnover can help fulfill the thermody-
namic requirements of a challenging reaction,69 but this
approach relies on efficient red-light absorbers that are
sufficiently robust under high irradiation densities. Moreover,
intersystem crossing leading to population of the photo-
sensitizer’s lowest-lying triplet excited state must be rapid, and
one usually faces the challenge that undesired nonradiative
relaxation processes become increasingly important the lower
the triplet energy gets.70 So far, mostly certain carefully
designed osmium,71−74 palladium,66,75−79 and platinum80−82
complexes fulfill the necessary requirements for red-light
absorbing triplet sensitizers, whereas base metal complexes
with the exception of some Cu(I) and Zn(II) com-
pounds83−85have remained underexplored in this context.86
[Mo(L3)3] satisfies all the necessary conditions for sensitized
red-to-blue light upconversion with 9,10-diphenylanthracene
(DPA) as the triplet annihilator. [Mo(L3)3] tolerates high
excitation densities of red light over several hours, enabling the
generation of a strong blue-photon flux over extended periods.
To illustrate this, a glass tube in which the photochemical
upconversion occurs served as the only irradiation source for a
separate, blue-light initiated photoreaction.
Figure 1. Molecular structures of ligands previously used for Cr(0)
and Mo(0) complexes (A)46−48 and the new ligand (B) yielding the
[Mo(L3)3] complex reported herein.
With the specific goal of targeting deep red luminescent
Mo(0) complexes, we made two significant changes to the
ligand design (Figure 1B): (1) replacing the central 6-
membered phenylene ring with a 5-membered thiophene
ring; (2) replacing the tert-butyl groups situated para to the
isocyanide groups with aryl substituents. These ligand
modifications seek to minimize steric restrictions associated
with the central ligand ring, thereby facilitating access to ligand
conformations with improved bite-angles, and extending the
effective ligand π-conjugation to shift MLCT absorption and
emission to the red.49 As a result, we obtained the homoleptic
complex, [Mo(L3)3], with photophysical properties reminis-
cent of those of [Os(bpy)3]2+, i.e., deep red 3MLCT
luminescence with an excited-state lifetime on the order of
50−90 ns in deaerated solution at room temperature and an
unusually high photorobustness. This discovery is relevant in
the context of using red excitation light for photochemical
reactions, light-harvesting,50 imaging, or phototherapeutic
purposes.51 Red light is less damaging than UV or blue
irradiation,52 can lead to more selective excitation of
photocatalysts in complex reaction mixtures,53 and can have
Our work suggests that a much wider scope of transition
metal compounds than what was long considered relevant
could be useful as red-light (MLCT) emitters and triplet
sensitizers, complementing recent insights gained in the area of
metal-free dyes and photosensitizers.87−95
a
Scheme 1. Synthesis of the Ligand L3 and the Homoleptic [Mo(L3)3] Complex
a
Reagents and conditions: (a) Pd(PPh3)4, K2CO3, dioxane/H2O, microwave, 90 °C, 97%; (b) TFA, CH2Cl2, 0 °C to rt, 94%; (c) NBu4Br3, THF, 0
°C, 85%; (d) acetic anhydride, HCOOH, 0 °C to rt, 80%; (e) PdCl2(dppf), xylene, 139 °C, 57%; (f) POCl3, N,N-diisopropylamine, CH2Cl2, 0 °C
to rt, 70%; (g) Na/Hg, THF, rt, 23%.
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RESULTS AND DISCUSSION
■
Synthesis and Infrared Spectroscopy. The synthesis of
ligand L3 started with the preparation of compounds 1 and 2
from commercially available precursors (Scheme 1). Specifi-
96
cally, 2-tert-butylaniline was brominated using NBu4Br3 and
its amino group subsequently Boc-protected to yield 1.97 1-
Bromo-3,5-dimethoxybenzene was Miyaura coupled with
bis(pinacolato)diboron to afford 2. Suzuki coupling of 1 and
2 afforded compound 3a, which was deprotected using
CF3COOH to yield the aniline 3b. Bromination of 3b with
NBu4Br3 occurred selectively in the ortho-position relative to
the amino group, resulting in 3c. Protection of the aniline with
formamide (to afford 3d) was necessary before efficient Stille
coupling with commercially available 2,5-bis(tri-n-
butylstannyl)thiophene was possible. Dehydration of the
coupling product 4 gave the final ligand L3 in 18% overall
yield based on the commercially available 2-tert-butylaniline
precursor. The [Mo(L3)3] complex was prepared by reacting 3
equiv of L3 with MoCl4(thf)298 using Na/Hg as reductant.43,46
As a powder, the [Mo(L3)3] complex can be handled under an
ambient atmosphere without noticeable degradation. In dilute
solutions, it is sensitive to dissolved oxygen (see SI page S6 for
details).
Figure 2. (A) Cyclic voltammogram of the [Mo(L3)3] Mo(0/I) redox
couple in deaerated THF containing 0.1 M TBAPF6 at 20 °C,
recorded with a scan rate of 200 mV/s. (B) Randles−Sevcik plot for
the Mo(0/I) redox couple from (A).
0.2 eV relative to that of [Mo(L2)3]. In the range between 0
and −2.0 V vs SCE, no redox waves are detectable (Figure S9),
and thus the reduction potentials of the attached L3 ligands
cannot be directly determined. Based on an energy of the
emissive MLCT excited state (EMLCT) of 1.94 eV (see below),
we estimate a potential of −1.84 V vs SCE for the Mo(0/I)
redox couple in the emissive MLCT excited state (Mo*(0/I))
(Figure S10). Thus, [Mo(L3)3] is expected to be a strong
photoreductant, although this aspect is not the focus of the
present study.
The CN stretching frequency (υCN) of isocyanides is
usually very susceptible to metal coordination,99−101 and this is
also the case here. In the IR spectrum of the free ligand, L3,
υCN is observed at 2112 cm−1, whereas in [Mo(L3)3] it is
shifted to 1930 cm−1 as a result of π-backbonding (Figure
S8).102 Furthermore, the respective IR absorption band is
substantially broader in the complex when compared to the
free ligand, possibly due to geometric isocyanide distortions as
a consequence of strong π-back-donation from the electron-
rich Mo(0) center. Due to the change in ligand design
incorporating now a central thiophene instead of a phenylene
unit between arylisocyanide moieties (Figure 1), we antici-
pated a significant change in metal coordination. Unfortu-
nately, we were unable to obtain single crystals of [Mo(L3)3]
to test this hypothesis by X-ray diffraction. However, π-
backbonding is significantly stronger in [Mo(L3)3] than in
[Mo(L2)3] based on the observable shift of υCN between free
ligand and metal complex, suggesting that L3 can adopt
conformations leading to better metal−ligand orbital overlap
and more ideal octahedral coordination of Mo(0) than L2.
Specifically, the free ligands L3 and L2 exhibit an identical
CN stretch frequency of 2112 cm−1,48 indicating that
Optical Spectroscopy. [Mo(L3)3] exhibits similar UV−vis
absorption bands as [Mo(L2)3],48 but all of them are shifted by
3000−8000 cm−1 to lower energies (solid black traces in
Figure 3). In the UV spectral region, the ligand-centered π−π*
electronic substituent effects have negligible influence on υCN
,
but upon coordination to Mo(0), υCN decreases by 182 cm−1
in the case of [Mo(L3)3] but only by 161 cm−1 in [Mo(L2)3].
The comparison between L3 and L2 is meaningful, because
these ligands both feature the same bulky tert-butyl
substituents in ortho-position to the isocyanide group (Figure
1).
Figure 3. UV−vis absorption spectra of (A) [Mo(L3)3] in
cyclohexane and (B) [Mo(L2)3] in n-hexane at 20 °C (solid black
traces). Luminescence spectra of (A) [Mo(L3)3] and (B) [Mo(L2)3]
in different deaerated solvents (red traces, see inset) at 20 °C.
Excitation occurred at 465 nm for [Mo(L3)3] and at 500 nm for
[Mo(L2)3]. The [Mo(L2)3] data is from ref 48. The dashed green
trace in (A) is the (arbitrarily scaled) excitation spectrum obtained by
monitoring the [Mo(L3)3] emission in deaerated toluene at 705 nm.
Electrochemistry. The cyclic voltammogram of the
[Mo(L3)3] complex in deaerated THF containing 0.1 M
TBAPF6 shows an oxidation wave at 0.1 V vs SCE (Figure 2A),
which is attributable to the Mo(0/I) redox couple. The linear
relationship between peak current and potential scan rate
(Figure 2B) indicates full reversibility of this metal-centered
redox process. In the previously investigated [Mo(L2)3]
complex the Mo(0/I) potential is approximately 0.2 V less
positive (−0.08 V vs SCE),48 suggesting that the largely metal-
centered (“t2g”-like) HOMO of [Mo(L3)3] is stabilized by ca.
absorption band shifts from 232 nm for the complex with the
m-terphenyl based ligand (L2) to 285 nm for the complex with
the new 2,5-diphenylthiophene based ligand (L3), indicating a
greater extent of π-conjugation in the latter. In the visible
region, the band maximum of the lowest MLCT absorption
shifts from 480 nm for [Mo(L2)3] to 550 nm in [Mo(L3)3].
The new ligand design furthermore entails a factor-of-2
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a
Table 1. Photophysical Parameters of Two Mo(0) Complexes and [M(bpy)3]2+ Reference Complexes (M = Os, Ru)
complex
λem/nm
τ/ns
ϕ
kr/s−1
knr/s−1
b
[Mo(L3)3]
720
723
587
620
56
60
1293
855
0.015
0.00462
0.203
2.6 × 105
1.8 × 107
c
c
[Os(bpy)3]2+
7.71 × 104
1.66 × 107
d
e
[Mo(L2)3]
[Ru(bpy)3]2+
0.062
7.25 × 104
1.1 × 106
See text for definitions of the individual parameters. In deaerated toluene at 20 °C; this work. In deoxygenated acetonitrile at 23 °C.105−107 In
deaerated toluene at 20 °C; from ref 48. Weighted average value for two conformers with different lifetimes (τ1 = 1110 ns (85%), τ2 = 2330 ns
(15%)).48 The calculation of radiative (kr) and nonradiative decay rate constants (knr) is not meaningful in this case, because the excited-state decay
is biexponential.
a
b
c
d
e
increase of the molar extinction coefficients of the visible
absorptions. The lowest electronic transitions are likely to have
mixed MLCT and π−π* (CN−C) character based on prior
computational studies of W(0) arylisocyanides,103 enhancing
their extinction coefficients. In the [Mo(L3)3] complex, the
lowest absorption band tails to nearly 680 nm. At 635 nm the
extinction coefficient ε635 is still 3340 M−1 cm−1, which makes
efficient photoexcitation with red light possible. The [Os-
(bpy)3]2+ complex, which has been widely used for red-to-blue
upconversion, has a similar molar extinction coefficient at that
Figure 4. (A) Transient absorption spectrum of 20 μM [Mo(L3)3] in
wavelength (ε635 nm ≈ 4000 M−1 cm−1),104 though in this case
deaerated toluene at 20 °C, recorded with a delay time of 20 ns
the relevant electronic transition is a formally spin-forbidden
following excitation at 532 nm with pulses of ca. 10 ns duration. The
signal was time-integrated over 200 ns. (B) Temporal evolution of the
singlet−triplet absorption. In [Mo(L3)3], excitation at 635 nm
occurs into a spin-allowed absorption band, yet the energy loss
associated with intersystem crossing (ISC) to the lowest triplet
state remains favorably small in this case (see below).
[Mo(L3)3] shows red luminescence at room temperature,
with band maxima (λem) at 705, 720, and 730 nm in
cyclohexane, toluene, and THF, respectively (red traces in
Figure 3A). Compared to [Mo(L2)3] under comparable
conditions,48 this corresponds to a red shift of ca. 3000
cm−1, in line with the MLCT absorption band shifts described
above. The observable emission solvatochromism is as
expected for MLCT states, in which the dipole moment is
larger than in the electronic ground state. The excitation
spectrum of the [Mo(L3)3] luminescence detected at 705 nm
(dashed green trace in Figure 3A) closely follows the UV−vis
absorption spectrum, indicating that higher lying excited states
ultimately lead to population of the lowest (emissive) excited
state. From an emission spectrum recorded in 2-methyl-THF
at 77 K, an MLCT energy (EMLCT) of 1.94 eV can be estimated
(Figure S12).
different signals from (A) as indicated in the inset.
attributed to the one-electron reduced ligand of the MLCT
state.48 All of these excited-state absorption features, MLCT
bleaches, and emission signals exhibit identical (single-
exponential) decay behavior (Figure 4B), yielding an MLCT
lifetime (τ) of 56 ns in deaerated toluene at 20 °C. This
relatively long lifetime is in line with the expected triplet
3
character of the excited state. The single-exponential MLCT
kinetics of [Mo(L3)3] are in contrast to the biexponential
decays of the previously studied [Mo(L2)3],48 which we
tentatively attribute to greater conformational flexibility of the
thiophene ring of L3 compared to the central benzene unit of
L2. Based on τ = 56 ns and ϕ = 0.015, the rate constants for
3
radiative (kr) and nonradiative MLCT deactivation (knr) in
deaerated toluene at room temperature are 2.6 × 105 s−1 and
1.8 × 107 s−1, respectively (Table 1).
The photoluminescence quantum yield (ϕ) of [Mo(L3)3] in
deaerated toluene at 20 °C is 0.015 (Table 1), approximately
13 times lower than that for [Mo(L2)3] under identical
conditions (ϕ = 0.203).48 Given the markedly lower MLCT
energy in [Mo(L3)3], this is unsurprising in light of the energy
gap law.70 In prior studies of Os(II)-based emitters, a decrease
in MLCT energy by 3000 cm−1 caused an increase of the rate
constant for nonradiative MLCT deactivation by a factor of ca.
30, lowering ϕ by roughly the same factor.105 For reference,
the emission quantum yield for [Os(bpy)3]2+ is 0.00462 in
deoxygenated CH3CN at 23 °C,105 roughly 3 times lower than
that for [Mo(L3)3] (Table 1).
The transient absorption spectrum recorded after pulsed
excitation of [Mo(L3)3] at 532 nm (Figure 4A) exhibits a
bleach in the region of the MLCT absorption bands with
(negative) peaks coinciding with those of the ground-state
absorption spectrum (475 and 560 nm in toluene). Also
observable is a negative signal around 720 nm due to emission,
and an excited-state absorption feature near 375 nm, which is
In addition to nonradiative relaxation directly from the
excited to the ground state as mentioned above, Ru(II)
polypyridine complexes typically exhibit thermally activated
nonradiative relaxation pathways involving the population of
MC states. While this aspect has received significant attention
in Ru(II) polypyridine complexes,108,109 it has been hitherto
neglected in most investigations of group 6 metal isocyanide
complexes. The temperature-dependent luminescence lifetime
data in Figure 5 allow us to address this issue for [Mo(L3)3].
Between 10 and 50 °C, the 3MLCT lifetime (τ) of [Mo(L3)3]
in deaerated toluene decreases by approximately a factor of 2,
as observed in Figure 5B (kobs = τ−1). It is well-established that
thermally activated nonradiative deactivation through low-lying
MC states is best described using a model in which the
3MLCT and 3MC states are thermally equilibrated.110 The kobs
values were therefore fitted using eq 1 (Figure 5B), which is
derived from a simple two-state thermally equilibrated model
as depicted in Figure 5C,110,111 and adequately models the
observed temperature-dependence of kobs
.
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use [Ru(bpy)3]2+ as a reference point for certain performance
factors. On the one hand, this is because Mo(0) and Ru(II) are
both isoelectronic second-row transition metals, and on the
other hand [Ru(bpy)3]2+ has been an important benchmark
compound in many previous investigations.113 Comparative
photostability studies were therefore performed with [Mo-
(L3)3] and [Ru(bpy)3]2+, using solutions with essentially
identical absorbance at the excitation wavelength, and
photoluminescence was monitored as a function of irradiation
time (Figure 6). Mainly for reasons of solubility, but also
Figure 5. (A) 3MLCT luminescence decays of [Mo(L3)3] in
deaerated toluene at different temperatures. In this TCSPC
experiment, excitation occurred at 475 nm, detection was at 710
nm. (B) Decay rate constants (kobs = τ−1) extracted from the data in
(A). The solid gray line is a fit with eq 1 to the experimental data,
yielding the parameters given in the inset. (C) Three-state model for
3MLCT excited state decay including thermal population of a nearby
MC state; kMLCT is the rate constant for MLCT decay directly to the
ground state (encompassing both radiative and nonradiative
contributions), and kMC is the rate constant for nonradiative MC
state decay to the ground state.
Figure 6. Relative photostability studies. Lower gray trace:
Normalized luminescence intensity of 96 μM [Ru(bpy)3]2+ at 620
nm in deaerated acetonitrile under continuous irradiation with a 500
mW laser at 532 nm. Upper gray trace: Normalized luminescence
intensity of 3 μM [Mo(L3)3] at 710 nm in deaerated toluene under
continuous irradiation with a 500 mW laser at 532 nm. The complex
concentrations were adjusted such that both samples had essentially
identical absorbance (OD approximately 0.08) at the excitation
wavelength (see SI page S30 for details). The red trace is an
exponential fit to the gray experimental data.
kobs = [kMLCT + kMC × exp(−ΔE/kB × T)]
because it is the preferred solvent for the upconversion studies
presented below, toluene was used for [Mo(L3)3], whereas the
reference compound was dissolved in acetonitrile, the most
typical solvent for [Ru(bpy)3](PF6)2. The excitation source
was a cw-laser with 500 mW output at 532 nm, which
irradiated the two solutions in common 1 cm quartz cuvettes
inside a fluorimeter. The time traces in Figure 6 represent the
luminescence intensities at (or near) the respective emission
maxima (710 nm for [Mo(L3)3], 620 nm for [Ru(bpy)3]2+),
normalized to the initial intensity at the start of the irradiation
period. Over the span of 4 h, the emission intensity of
[Ru(bpy)3]2+ decreases to less than 10% of its initial value,
closely following an exponential function from which a half-life
for photodegradation (τ1/2) of 74 min can be determined. The
luminescence intensity of [Mo(L3)3] decreases only to 94% of
its initial value in the same time period. From the two data sets
in Figure 6 we calculate that the initial photodegradation rate
(concentration change per unit time) of [Mo(L3)3] in toluene
is roughly a factor of 1300 lower than that for [Ru(bpy)3]2+ in
acetonitrile under the high-power excitation conditions
employed here (SI, page S30). Direct comparison of both
complexes in the same solvent would be needed to make a
truly quantitative statement concerning their relative inherent
photostabilities, but the very disparate solubility characteristics
of charge-neutral [Mo(L3)3] and dicationic [Ru(bpy)3]2+
prohibit this. The main point here is the remarkably high
photorobustness of [Mo(L3)3] in toluene, the preferred solvent
for the upconversion studies presented below. Based on our
data, the quantum yield for photodegradation of [Mo(L3)3] in
toluene is only ca. 10−7 (SI page S31).
/[1 + exp(−ΔE/kB × T)]
(1)
In this simplistic but common picture, kMLCT is the total
3
(radiative plus nonradiative) MLCT excited state decay rate
constant for direct relaxation to the ground state, kMC is the rate
constant for nonradiative relaxation from the thermally
populated MC state, ΔE is the energy difference between
the MC and 3MLCT state, and kB is Boltzmann’s constant. The
best (unconstrained) fit is obtained with kMLCT = 6.6 × 106 s−1,
kMC = 1.0 × 1010 s−1, and ΔE = 1300 cm−1 (solid gray line in
Figure 5B). An underlying assumption here is that the
observed temperature dependence is caused by varying relative
3
populations of only two excited states (one MLCT and one
MC state), whereas the kMLCT and kMC parameters themselves
are temperature-independent within the investigated range.
When further assuming identical degeneracies for the MLCT
3
and MC states, the Boltzmann population of the MC state at
298 K is approximately 0.2% of the total excited-state
population. The rate for nonradiative depopulation through
the MC channel then becomes 0.002 × kMC = 2 × 107 s−1,
which is on the same order of magnitude as the experimentally
determined knr value (Table 1). This crude approximation
3
therefore suggests that nonradiative MLCT relaxation via the
MC channel is a major decay path for [Mo(L3)3]. For
comparison, [Ru(bpy)3]2+ and many of its closely related
derivatives have ΔE values on the order of 3500 cm−1 (0.43
eV),111,112 and consequently the MC state plays a smaller role
for 3MLCT depopulation in these complexes than in
[Mo(L3)3].
Photostability. Although the photophysical properties of
Triplet−Triplet Energy Transfer to DPA. With its triplet
[Mo(L3)3] resemble those of [Os(bpy)3]2+, it is meaningful to
excited state at 1.77 eV,114 DPA should be able to quench the
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3MLCT luminescence of [Mo(L3)3] by triplet−triplet energy
transfer (TTET). This is indeed the case (Figure 7A), and
10 mM DPA at various excitation densities results in the series
of luminescence spectra shown in the left part of Figure 8A.
Figure 7. TTET from 3MLCT-excited [Mo(L3)3] to DPA. (A)
Decays of the 3MLCT luminescence of 10−12 μM [Mo(L3)3] at 710
nm in deaerated toluene in absence (black trace) and in the presence
of different DPA concentrations (colored traces). Excitation occurred
at 532 nm with 20 mJ laser pulses of ca. 10 ns duration. (B) Stern−
Volmer plot resulting from the data in (A). (C) Transient absorption
spectrum recorded after 532 nm excitation of 12 μM [Mo(L3)3] in
the presence of 50 mM DPA in deaerated toluene at 20 °C. The pulse
duration was ca. 10 ns, the spectrum was recorded with a delay of 300
ns and time-integrated over an interval of 200 ns.
Figure 8. sTTA-UC. (A) Left: Delayed fluorescence after selective
excitation of 11 μM [Mo(L3)3] in the presence of 2.5 mM DPA in
deaerated toluene at 20 °C. Various power densities between 3 and 22
3
W/cm2 were used for excitation at 635 nm. Right: Prompt MLCT
emission (scaled by a factor of approximately 10−2) of 10 μM
[Mo(L3)3] in deaerated toluene at 20 °C, containing no DPA but
measured under otherwise strictly identical conditions as the delayed
fluorescence spectra (see SI for details). (B) DPA fluorescence spectra
as a function of DPA concentration (solid traces): 0.1 μM (blue), 2.5
mM (black), 10 mM (green), 50 mM (red). The dashed blue trace is
an arbitrarily scaled absorption spectrum of DPA to help visualize the
inner filter effect. (C) Upconversion efficiency as a function of
excitation power density, as determined from eq 2 and the data in (A)
for 2.5 mM DPA (black triangles) and an analogous data set obtained
at 10 mM DPA concentration (green triangles). (D) Excitation power
dependence of the delayed fluorescence intensity from (A) (2.5 mM
DPA (black triangles)) and an analogous data set obtained at 10 mM
DPA (green triangles). The gray triangles show the excitation power
Stern−Volmer analysis of the DPA concentration-dependent
3MLCT luminescence lifetime data in deaerated toluene
(Figure 7B) yields a Stern−Volmer quenching constant
(KSV) of 11.3 M−1. Transient absorption spectroscopy
confirms that triplet-excited DPA (3*DPA) is formed in this
quenching process (Figure 7C), and there is no evidence for
the formation of DPA radical ions;115 hence, photoinduced
electron transfer between [Mo(L3)3] and DPA is unimportant
despite the high reducing power of photoexcited [Mo(L3)3]
(−1.84 V vs SCE).114 Based on τ0 = 54 ns from the
luminescence lifetime measurement by TCSPC (the value of
56 ns in Table 1 was determined by transient absorption
spectroscopy) and KSV = 11.3 M−1, a rate constant (kTTET) of
(2.1 0.2) × 108 M−1 s−1 is determined for TTET, which is
roughly a factor of 50 below the diffusion limit for bimolecular
reactions in toluene at 20 °C (1.1 × 1010 M−1 s−1).114 At the
highest DPA concentration employed (50 mM, approaching
the solubility limit), the 3MLCT lifetime (τ) is reduced to 34.6
ns, and thus the TTET efficiency (ηTTET = 1 − τ/τ0) becomes
0.36, whereas for a DPA concentration of 10 mM, ηTTET drops
to 0.12. Rate constants for TTET can typically approach the
diffusion limit when a driving force (−ΔGTTET) of ca. 0.2 eV is
reached.116 In our case, we estimate ΔGTTET ≈ −0.17 eV, yet
kTTET is 50 times below the diffusion limit. We speculate that
this could be due to the bulkiness of [Mo(L3)3], which might
impede the formation of strong donor−acceptor orbital
overlaps in the collisional encounter complex between
[Mo(L3)3] and DPA.
3
dependence of the [Mo(L3)3] MLCT emission from the data set in
(A).
The prominent band between 25 000 and 17 000 cm−1 is due
to delayed fluorescence from DPA, while the band below
15 500 cm−1 is the unquenched 3MLCT emission of
[Mo(L3)3] in the absence of DPA but under otherwise strictly
identical conditions. The latter spectra serve as reference
points for the estimation of the upconversion efficiency, as
discussed in detail further below. In time-resolved experiments
using pulsed excitation at 532 nm, the DPA fluorescence
decays on a time scale of several hundred microseconds in
deaerated toluene at 20 °C, reflecting the 153 μs lifetime of the
lowest DPA triplet excited state (3*DPA) as determined by
transient absorption spectroscopy (Figure S15). By contrast,
the 3MLCT emission of [Mo(L3)3] (τ0 = 54 ns; see above) has
decayed essentially completely after 300 ns (Figure 7A). The
3
observation of a minor residual MLCT luminescence signal
beyond 300 ns (Figure S16) indicates that there is either some
reverse TTET from 3*DPA to [Mo(L3)3] or some
reabsorption of DPA fluorescence by [Mo(L3)3] (or a
combination of both), but these undesired processes are
evidently very inefficient.
Photochemical Upconversion. Photoexcitation of [Mo-
(L3)3] (11 μM) with a cw-laser at 635 nm in the presence of
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The spectral shape of the DPA fluorescence is concentration
dependent, and this is important for determination of the anti-
Stokes shift. Upon direct excitation in highly dilute solution
(solid blue trace in Figure 8B), the so-called 0−0 band at 405
nm is more prominent than all following progression members
in a skeletal vibration mode and as such represents the
emission band maximum. However, at the concentrations
relevant to upconversion, the emission band maximum is at
430 nm due to an inner-filter effect attenuating the 0−0 band
(black, green, and red traces in Figure 8B). Thus, based on an
excitation wavelength of 635 nm and an emission band
maximum at 430 nm, an anti-Stokes shift of 0.93 eV is
obtained. This value compares favorably to the many sTTA-
UC systems exhibiting anti-Stokes shifts in the range 0.4−0.8
eV.76,117 Two recent studies claimed to observe greater anti-
Stokes shifts with DPA (1.08 eV,81 1.14 eV104) despite very
similar excitation wavelengths (665/663 nm). However, these
estimates were based on the DPA fluorescence band maximum
in very dilute solution, even though this is not the band
maximum under upconversion conditions and consequently
leads to an overestimation of the anti-Stokes shift. Using
similar analysis, we would obtain a value of 1.11 eV. Other
studies reported anti-Stokes shifts between 0.97 and 1.28 eV in
systems with anthracene- and perylene-based annihila-
tors,74,118,119 and in a recently disclosed lanthanide-based
upconversion system, an anti-Stokes shift of just less than 1 eV
was found.120 Against this background, it seems fair to state
that the anti-Stokes shift of our [Mo(L3)3]/DPA combo is
among the largest reported to date.
DPA concentration of 10 mM, the highest achievable
upconversion quantum yield is 0.0037 at an excitation power
density of 22 W/cm2, but under these conditions we were
unable to reach the threshold power density (Ith) after which
ϕUC reaches its true upper limit. However, when increasing the
DPA concentration from 10 to 50 mM, ϕUC improves to 0.018
(Figure S21), mainly because the TTET efficiency (ηTTET
)
between MLCT-excited [Mo(L3)3] and DPA increases from
0.12 to 0.36 (see above). Thus, the achievable upconversion
quantum yield of the [Mo(L3)3]/DPA combination ap-
proaches that of many sTTA-UC systems with precious
metal-based sensitizers,76 including some of the recently
reported systems with record anti-Stokes shifts around 1
eV.81,104,118−120 For instance, two Os-based systems yielded
ϕUC = 0.050−0.055 (with a theoretical limit of 100%; in this
case our ϕUC would be 0.036),104,118 whereas Pd- and Pt-based
systems gave ϕUC = 0.21−0.27.81,119
3
Given the need for high excitation densities for efficient
sTTA-UC in most systems, the issue of long-term photo-
stability under upconversion conditions is an important aspect.
The inherent photostability of [Mo(L3)3] under green light
excitation is remarkably high (Figure 6), and the [Mo(L3)3]/
DPA combo is also very robust under red light excitation
(Figure 9). Using an excitation power density of 1.25 W/cm2
Integrated emission intensities from excitation power-
dependent measurements with a 635 nm cw-laser are shown
3
in Figure 8D. Expectedly, the MLCT luminescence intensity
increases linearly with increasing excitation power (gray
triangles), whereas the upconverted DPA fluorescence
intensity is best fitted to power functions yielding exponents
between 1.38 and 1.50 (green and black triangles), depending
on exact conditions. The significant deviation from quadratic
power dependence is likely a manifestation of the fact that the
system approaches the strong annihilation limit, in which the
rate constant for TTA exceeds the rate constant for triplet−
triplet energy transfer (kTTA > kTTET).121 This hypothesis is
supported by the finding of a higher exponent (n) at lower
DPA concentration (n = 1.50 for 2.5 mM DPA, black triangles
in Figure 8D, versus n = 1.38 for 10 mM DPA, green triangles).
As noted in the previous section, kTTET is roughly a factor of 50
below the diffusion limit, and this can help explain why the
strong annihilation limit is apparently approached easily with
the [Mo(L3)3]/DPA couple.
The quantum yield for upconversion (ϕUC) was determined
with eq 2,117 using as input data those from Figure 8A, i.e. the
[Mo(L3)3]/DPA combo and a reference solution containing
only [Mo(L3)3] but no DPA. In that equation, AbsUC and
Absref represent the absorbance of the two solutions at 635 nm,
whereas IUC and Iref stand for the integrated DPA and 3MLCT
luminescence intensities of the two different solutions at a
given excitation density. ϕref is the 3MLCT luminescence
quantum yield of [Mo(L3)3]. Equation 2 is formulated such
Figure 9. Photostability under upconversion conditions. Delayed
fluorescence intensity emitted at 430 nm by a deaerated toluene
solution of 13 μM [Mo(L3)3] and 2 mM DPA in the course of cw-
laser irradiation at 635 nm with a power density of 1.25 W/cm2.
at 635 nm to irradiate a deaerated toluene solution containing
13 μM [Mo(L3)3] and 2 mM DPA at room temperature, the
upconversion luminescence intensity at 430 nm was monitored
as a function of time. Over the first 6 h, the signal decreases
more or less linearly to 40% of its initial value, and over the
following 10 h decays further to 9%. Thus, the upconversion
system seems considerably less photorobust than [Mo(L3)3]
alone, though it should be kept in mind that excitation
conditions to obtain the data sets in Figures 6 and 9 were
different. Nevertheless, it seems plausible that the long-lived
triplet excited state of DPA is particularly susceptible to
undesired side reactions, and that the annihilator is therefore
limiting the long-term performance of the overall system.
Photochemistry with Upconverted Light. In several
recent studies, sTTA-UC was exploited to drive a photo-
chemical reaction.53,56,63,65,69,123−125 Performing a blue- or
green-light dependent reaction in the same flask in which a
sensitizer is excited with red light and the annihilator performs
upconversion is a very elegant approach. Here, we aimed to
explore a different concept, in which the upconversion and the
blue-light-dependent photoreaction occur in spatially separate
reaction vessels. Given the robustness of the [Mo(L3)3]/DPA
system, we were curious whether its blue upconversion output
could be sufficiently intense and durable to serve as a “blue
that a maximum value of 0.5 (i.e., 50%) can result for
117,122
ϕUC
.
ϕUC = ϕ × (Absref /AbsUC) × (IUC/Iref)
(2)
ref
The outcome for two different annihilator concentrations
and different excitation densities is shown in Figure 8C. At a
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Figure 10. (A) Experimental setup for the photochemical isomerization of stilbene with upconverted light. The specific elements are (1) a red (635
nm) diode laser, (2) a beam expander used backward as an optical telescope, (3) a cooling bath with water, and (4) a quartz cuvette with the two
spatially separate solutions, as illustrated in (B). A sealed NMR tube containing 13 μM [Mo(L3)3] and 60 mM DPA in deaerated toluene is
immersed into the cuvette containing 0.3 mM [Ru(bpy)3]2+ and 17 mM trans-stilbene in deaerated CD3CN. Red-to-blue upconversion inside the
NMR tube is visible by naked eye (right-hand side of (B)). (C) Operating principle of this setup: 635 nm excites exclusively [Mo(L3)3] (penetrates
the surrounding solution containing [Ru(bpy)3]2+), followed by intersystem crossing (ISC) and TTET to DPA. Triplet-excited DPA undergoes
TTA and emits blue upconverted fluorescence. That 430 nm light passes across the NMR tube glass and excites [Ru(bpy)3]2+ in the surrounding
solution. Following ISC and TTET to trans-stilbene, the substrate isomerizes to cis-stilbene.
lamp” for an NMR-scale reaction. Considering that many
commercially available LEDs for photoredox catalysis have
peak radiant fluxes on the order of 0.5 W/nm, it is not obvious
that a photochemical upconversion system will provide
sufficient photon flux and durability to drive a photochemical
reaction requiring irradiation over several hours.
the NMR tube, but no DPA, and irradiating at 635 nm for 17
h, only 5% of the trans-stilbene underwent isomerization
(Figure 11C). This series of experiments clearly demonstrates
the viability of the concept illustrated in Figure 10C.
The turnover number (TON) for the reaction mixture from
Figure 11B is 1095 after 17 h of irradiation, calculated with
respect to the [Mo(L3)3] concentration (see SI pages S28−
S29). According to the photostability data in Figure 9, the
performance of the upconversion system decreases by 60%
over the first 6 h. Consequently, when replacing the sTTA-UC
system in the cuvette of Figure 10B after 5 h with a fresh
solution of [Mo(L3)3] and DPA, the overall reaction rate is
improved, resulting in a similar TON (968) already after 10 h
instead of 17 h.
The main purpose of the experiment in Figure 10 was to
showcase the high performance and long-term robustness of
the upconversion system based on an Earth-abundant metal
complex, but the concept of using a photochemical
upconversion system to drive a secondary light-dependent
process in a separate reaction vessel might become interesting
in broader contexts.128−133 In particular, the spatial separation
allows the formation of high-energy triplets from low-energy
input light, instead of the high-energy singlets usually formed in
sTTA-UC. In a one-pot reaction, the high-energy triplets (in
our case the 3MLCT excited state of [Ru(bpy)3]2+ at 2.12 eV)
are expected to be quenched by the lower lying triplet states of
To test this concept, a solution of 13 μM [Mo(L3)3] and 60
mM DPA in deaerated toluene was sealed in an NMR tube,
which was immersed into a cuvette containing 0.3 mM
[Ru(bpy)3]2+ and 17 mM trans-stilbene in deaerated CD3CN
(through an airtight rubber septum) as shown in Figure 10B.
The [Mo(L3)3] complex in the NMR tube was excited with a
laser beam at 635 nm (Figure 10A), leading to red-to-blue
upconversion that is detectable by the naked eye (right part of
Figure 10B). The red excitation light easily penetrates through
the surrounding [Ru(bpy)3]2+/stilbene solution in the cuvette,
because the optical density of this reaction mixture at 635 nm
is negligible.126 However, the upconverted blue light is readily
absorbed by [Ru(bpy)3]2+ (ε430 ≈ 14000 M−1 cm−1,126 present
at 0.3 mM) in the cuvette surrounding the NMR tube.
Intersystem crossing (ISC) and TTET lead to triplet-excited
trans-stilbene, which can isomerize to cis-stilbene (Figure
10C).127 After 17 h of irradiation at 635 nm, the yield of cis-
stilbene was 50% (Figure 11B), while in the absence of light no
conversion occurred at all (Figure 11A). When using a toluene
solution containing the same concentration of [Mo(L3)3] in
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in the case of Os(II) polypyridines, where red-light excitation
for upconversion purposes occurs into a formally spin-
forbidden singlet−triplet transition, [Mo(L3)3] can be excited
into a spin-allowed 1MLCT absorption band at 635 nm (ε635
=
3340 M−1 cm−1), and there is very little energy loss (0.012 eV)
3
upon relaxation to the MLCT state at 1.94 eV. The anti-
Stokes shift of 0.93 eV for the [Mo(L3)3]/DPA couple is
among the largest reported to date, and the upconversion
quantum yield of 0.018 is similar to some Os(II)-based
systems. The photorobustness under upconversion conditions
with intense (500 mW) red diode laser excitation is remarkable
and permits continued formation of blue output light over
several hours (Figure 9). The upconverted blue photon flux is
sufficiently high to drive a blue-light-dependent sensitized
photoisomerization reaction in a separate reaction vessel
(Figure 10). This concept of a photochemical upconversion
system serving as a light source for a spatially separate reaction
allows the formation of high energy triplet excited states from
low energy triplets, whereas under ordinary sTTA-UC
conditions only high energy singlets are formed. This opens
new perspectives in multiphoton excitation chemistry.69
Our study illustrates that research on Earth-abundant
alternatives for precious metal-based systems has much
potential for unexpected discoveries and fundamentally new
insights in photophysics and photochemistry, resonating with
recent studies in this field.16,17,28,29,38,143 This aspect seems
equally important as the obvious economic and sustainability
arguments associated with replacing precious metal complexes
such as those based on Ru(II) and Os(II) with Earth-abundant
transition metal compounds.
Figure 11. 1H NMR spectra for the stilbene photoisomerization with
upconverted light. A reaction mixture comprised of 0.3 mM
[Ru(bpy)3]2+ and 17 mM trans-stilbene in CD3CN was monitored
under three different conditions in the reaction setup of Figure 10A/
B: (A) The red laser remained switched off and the mixture stood in
the dark for 17 h. (B) The upconversion system in the NMR tube
comprised of 13 μM [Mo(L3)3] and 60 mM DPA was irradiated at
635 nm for 17 h. (C) The NMR tube contained only 13 μM
[Mo(L3)3] (but no DPA) and was irradiated at 635 nm for 17 h.
the upconversion sensitizer (here the [Mo(L3)3] complex with
3
its lowest MLCT state at 1.94 eV). Thus, the concept in
Figure 10 might become useful for thermodynamically very
challenging triplet photoreactions, and this seems noteworthy
given the recent surge of interest in energy transfer
catalysis.3,134−142
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c12805.
CONCLUSIONS
■
Synthetic protocols and characterization data, descrip-
tion of equipment and methods, supplementary electro-
chemical and spectroscopic data, details to upconversion
quantum yield determination, details to photochemical
experiments and photostability studies (PDF)
The incorporation of a thiophene bridging element and
peripheral aryl substituents extending the overall π-conjugation
of a chelating diisocyanide ligand (Scheme 1) gave access to a
homoleptic Mo(0) complex emitting at more than 120 nm
longer wavelengths than previously reported emissive Mo(0)
complexes (Figure 3).46,48 The photophysical properties of the
latter resembled those of [Ru(bpy)3]2+, whereas the new
[Mo(L3)3] complex behaves more similarly to [Os(bpy)3]2+
concerning emission color, lifetime, and quantum yield. Thus,
the diisocyanide chelate ligand design offers a tuning range that
is comparable to a change in metal for homoleptic
tris(bipyridine) complexes.
AUTHOR INFORMATION
■
Corresponding Author
Oliver S. Wenger − Department of Chemistry, University of
Basel, 4056 Basel, Switzerland; orcid.org/0000-0002-
0739-0553; Email: oliver.wenger@unibas.ch
The [Mo(L3)3] complex can be handled under an ambient
atmosphere as a powder, and in deaerated toluene solution
under photoirradiation it is remarkably robust (Figure 6).
Temperature-dependent studies indicate that the thermal
Authors
Jakob B. Bilger − Department of Chemistry, University of
Basel, 4056 Basel, Switzerland
Christoph Kerzig − Department of Chemistry, University of
Basel, 4056 Basel, Switzerland; orcid.org/0000-0002-
1026-1146
Christopher B. Larsen − Department of Chemistry, University
of Basel, 4056 Basel, Switzerland; orcid.org/0000-0003-
3006-2408
3
population of MC states from the luminescent MLCT state
can represent an important pathway for nonradiative relaxation
in zerovalent group 6 metal complexes (Figure 5), from which
one may conclude that future diisocyanide ligand designs
should aim at further increasing the ligand field strength in
3
order to maximize the MLCT-MC energy gap.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12805
The prior studies of photoactive Mo(0) complexes have
focused largely on their use as strong reductants in photoredox
catalysis,46,48 whereas the present work shows that [Mo(L3)3]
is well suited for red-to-blue upconversion via triplet−triplet
annihilation with 9,10-diphenylanthracene (Figure 8). Unlike
Notes
The authors declare no competing financial interest.
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Cyclic (amino)(aryl)carbenes enter the field of chromophore ligands:
expanded π system leads to unusually deep red emitting CuI
compounds. J. Am. Chem. Soc. 2020, 142, 8897−8909.
ACKNOWLEDGMENTS
■
This work was funded by the Swiss National Science
Foundation through Grant Number 200021_178760 and
through the NCCR Molecular Systems Engineering. C.K.
acknowledges a Novartis University of Basel Excellence
Scholarship for Life Sciences by the Research Fund of the
University of Basel. This work is dedicated to Professor
Wolfgang Kaim on the occasion of his 70th birthday.
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