Effects of Ligand Substitution on the Optical and Electrochemical Properties of (Pyridinedipyrrolide)zirconium Photosensitizers

Properties of (Pyridinedipyrrolide)zirconium Photosensitizers

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Eﬀects of Ligand Substitution on the Optical and Electrochemical

Yu Zhang, Dylan C. Leary, Anne M. Belldina, Je ﬀ rey L. Petersen, and Carsten Milsmann*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c02343

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sı Supporting Information

ABSTRACT: A series of seven bis(pyridinedipyrrolide)zirconium

R 2−

complexes, Zr( PDP ) , where [ PDP ]

is the doubly

deprotonated form of [2,6-bis(5-R -3-R -1H-pyrrol-2-yl)pyridine],

were prepared and characterized in solution by NMR, UV/vis

absorption, and emission spectroscopy and cyclic voltammetry.

The molecular structures were determined by single-crystal X-ray

crystallography. All complexes exhibit remarkably long emission

lifetimes (τ = 190−576 μs) with high quantum eﬃciencies (Φ

.10−0.38) upon excitation with visible light in a benzene solution. The substituents on the pyrrolide rings were shown to have

signiﬁcant eﬀects on the photoluminescence and electrochemical properties of these compounds. The R substituents (R = H, Me,

Ph, or C F ) show only limited eﬀects on the absorption and emission proﬁles of the complexes but allow systematic tuning of the

ground- and excited-state redox potentials over a range of almost 600 mV. The R substituents (R = H, Me, Ph, or 2,4,6-Me Ph)

inﬂuence both the optical and electrochemical properties through electronic eﬀects. Additionally, the R substituents have profound

consequences for the structural ﬂexibility and overall stability of the compounds. Distortions of the Zr(PDP) core from idealized

D symmetry in the solid state can be traced to the steric proﬁles of the R substituents and correlate with the observed Stokes shifts

for each compound. The complex with the smallest ligand system, Zr( PDP ) , coordinates two additional solvent molecules in a

tetrahydrofuran (THF) solution, which allowed the isolation of photoluminescent, eight-coordinate Zr( PDP ) (THF) . The

photoredox catalytic dehalogenation of aryl iodides and aryl chlorides using the most reducing derivative, Zr( PDP ) , highlights

the potential of Zr(PDP) photosensitizers to promote challenging reductive transformations under mild conditions upon excitation

with green light.

INTRODUCTION

ties can be more readily tuned through systematic ligand

modiﬁcations using the versatile toolboxes of synthetic organic

and coordination chemistry.

■

Transition-metal photosensitizers play an important role in a

wide range of chemical applications. Dye-sensitized solar

1−4

5,6

One generally accepted design principle for transition-metal

photosensitizers is rigidiﬁcation of the ligand framework. The

reduction of nonradiative decay pathways resulting from

vibrational modes can result in signiﬁcant improvements of

cells

and organic light-emitting diodes often rely on the

ability of molecular inorganic chromophores to convert light

into electrical energy (and vice versa) and are critical

components of a sustainable and eﬃcient energy economy.

At the same time, light-to-chemical energy conversion using

transition-metal complexes is central to the rapidly expanding

16,17

the luminescence quantum yields and excited-state lifetimes.

While this approach has been studied for a variety of

transition-metal chromophores, it is most dramatically

reﬂected in the optical properties of copper(I) bisphenanthro-

line complexes, in which long-lived luminescence is enabled

through the introduction of sterically demanding substituents

−15

ﬁelds of photoredox catalysis

and solar fuel generation,

which have the potential to revolutionize the production of

ﬁne and commodity chemicals. Finally, the ability of

photosensitizers to generate highly reactive species under

otherwise mild conditions through irradiation with visible light

at high temporal and spatial resolution is exploited in

that prevent excited-state Jahn−Teller ﬂattening from tetrahe-

20−23

dral to square-planar geometry.

A second concept, most

photodynamic therapy.

relevant to ﬁrst-row transition-metal photosensitizers, is the

Because each application within this wide range of

technologies has its distinct set of performance critical

parameterse.g., speciﬁc absorption or emission proﬁles,

range of redox potentials, or lifetime and quantum yield

demandscareful control over the properties of photo-

sensitizers is crucial. In this aspect, molecular transition-

metal photosensitizers oﬀer a distinct advantage over semi-

conductor or other solid-state materials because their proper-

Special Issue: Light-Controlled Reactivity of Metal

Complexes

Received: August 6, 2020

XXXX American Chemical Society

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Scheme 1. Synthetic Routes to New H₂^R¹PDP^RDerivatives

suppression of detrimental metal-centered (MC) excited states,

which similarly improves the luminescence quantum yields and

results in longer lifetimes. This approach has been shown for

chromium(0) isocyanide complexes featuring metal-to-ligand

trimethylphenyl substituents on the pyrroles are replaced by

much smaller methyl substituents. The increased stability of

Mes

Zr( PDP ) allowed detailed photophysical studies that

revealed that the long emission lifetime of Zr(PDP)₂

complexes can be attributed to thermally activated delayed

ﬂuorescence (TADF).

24,25

charge-transfer (MLCT) excited states,

complexes operating via d−d excitation,

iron(III) complexes exhibiting MLCT and ligand-to-metal

chromium(III)

and iron(II) and

6−28

In this work, we describe the inﬂuence of pyrrole

substitution at the 3 and 5 positions of these N-heterocycles

on the optical and electrochemical properties of Zr(PDP)₂

complexes. Solid-state molecular structures obtained by single-

crystal X-ray diﬀraction show a major inﬂuence of the

substituents at the 5 position of the pyrroles on the overall

geometry of the complexes, which results in considerable

changes to the optical properties. In contrast, substitution at

the 3 position produces only minor alterations in the

absorption and emission proﬁles but allows systematic tuning

9−32

charge-transfer (LMCT) states, respectively.

Aside from structural modiﬁcations, the introduction of

electron-donating or -withdrawing substituents into the ligand

scaﬀold can have a direct inﬂuence on the electronic structure

of the photosensitizer, which ideally allows systematic control

over the absorption and emission proﬁles and electron-transfer

33−35

properties in the ground and excited states.

The inﬂuence

of the substituents on the frontier molecular orbitals can be

either indirect through ligand-ﬁeld eﬀects on the metal d

orbitals (for MLCT, LMCT, or MC states) or direct if one or

both of the molecular orbital(s) involved in the corresponding

transition exhibit signiﬁcant ligand character. The latter is the

case for complexes with MLCT, LMCT, or ligand-to-ligand

charge-transfer excited states.

of the redox properties of Zr(PDP) complexes over a range of

more than 0.5 V.

RESULTS AND DISCUSSION

■

Ligand Synthesis. For unambiguous identiﬁcation of the

diﬀerent ligands and their resulting zirconium complexes

throughout this study, we have adopted the nomenclature

Recent work in our group has focused on the development

of photoluminescent zirconium complexes with pyridinedipyr-

36−38

rolide (PDP) ligands.

The bis-ligand complexes Zr-

PDP , where R^aⁿ^d^R2 represent the forward- and

(

PDP) exhibit remarkably long excited-state lifetimes that

allow facile photoinduced single-electron transfer under

photoredox catalytic conditions. These compounds expand

the limited pool of photosensitizers utilizing excited states with

backward-facing substituents, relative to coordination to the

metal center, at the 5 and 3 positions of the pyrrole rings,

respectively. A small library of novel PDP ligand precursors,

39−49

high LMCT character

and demonstrate the utility of more

PDP , was prepared following the synthetic approaches

Earth-abundant early transition metals in photochemical

applications, which are otherwise dominated by precious

outlined in Scheme 1. Three new ligand precursors,

PDP^C, were prepared in

6 5

PDP , H

PDP , and H

5,50,51

metal chromophores.

Initial investigations of the ligand

moderate to good yields following synthetic protocols similar

Mes

Ph36

substituent eﬀects resulted in the discovery of Zr( PDP )

to the ones previously reported for H PDP

H₂PDP

and

Mes

Ph52

based on the sterically demanding ligand precursor 2,6-bis[5-

using a two-step one-pot approach consisting

(

2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl]pyridine

of a Stetter reaction followed by Paal−Knorr synthesis. This

approach relies on the straightforward access to a variety of

α,β-unsaturated ketones and allows rigorous control over the

positioning of the introduced 3,5-substituents. The synthesis of

PDP derivatives with 5H-substitution is not amenable to these

Mes

Ph 38

H₂PDP ).

This complex showed a substantially

improved photoluminescence quantum yield and markedly

higher stability under ambient conditions compared to the

initially reported analogue Zr( PDP ) , in which the 2,4,6-

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Scheme 2. Synthesis of New Zr( PDP ) Complexes

reaction conditions because of selectivity issues between 2,6-

pyridinedicarboxaldehyde and the required α,β-unsaturated

aldehyde during the Stetter reaction. Instead, the substituent-

( PDP ) (THF) proved to be surprisingly labile and were

2 2

quantitatively removed by repeated coevaporation with

aromatic solvents or heating of the complex under a vacuum

in the solid state to yield the desired six-coordinate complex

free PDP derivative H PDP was readily obtained via

Suzuki−Miyaura coupling of commercially available 2,6-

dibromopyridine and N-Boc-2-pyrroleboronic acid followed

by thermal deprotection. Finally, PDP derivatives with

identical 3,5-pyrrole substituents can be prepared through

the condensation of symmetric 1,3-diketones with 2,6-

pyridinedimethanamine, as reported by Caulton and co-

Zr( PDP ) . Alternatively, direct synthetic access to Zr-

( PDP ) in excellent yield is available through the reaction

of H PDP and 0.5 equiv of tetrabenzylzirconium in benzene.

This reaction mirrors our recently developed synthesis for

Zr( PDP ) from H PDP and tetrabenzylzirconium at

high temperature but proceeds readily at ambient temper-

ature because of the lower steric proﬁle of the PDP ligand.

As expected, the addition of excess THF to Zr( PDP )

resulted in the formation of Zr( PDP ) (THF) , demonstrat-

ing complete reversibility of THF coordination (Scheme 2).

All complexes were characterized by H and C{ H} NMR

spectroscopy and exhibit resonances consistent with D₂_d-

symmetric structures and magnetically equivalent PDP ligands

on the NMR time scale in solution. This includes the eight-

coordinate complex Zr( PDP ) (THF) , indicating that

exchange of the labile THF ligands is rapid on the NMR

time scale.

Mes

workers. This method allowed the isolation of H₂PDP ,

albeit in poor yield.

Complex Synthesis. Following our previously reported

2 2

Ph 36

methodology for Zr( PDP ) , salt metathesis between the

lithium salts of the in situ generated, doubly deprotonated

R 2−

ligands, [ PDP ] , and 0.5 equiv of ZrCl in tetrahydrofuran

(

THF) furnished Zr( PDP ) , Zr( PDP ) , Zr-

2 2

6 5

PDP ) , and Zr( PDP ) in good yields (Scheme 2).

The analogous reaction using H PDP did not yield

2 2

Zr( PDP ) but instead provided the eight-coordinate

complex Zr( PDP ) (THF) . This result presents the ﬁrst

indication that the substituents in the R positions have a

Structural Characterization. All new complexes were

signiﬁcant impact on the coordination geometry and ligand

environment around the central zirconium ion because no

indication for THF coordination or expansion of the six-

coordinate ligand sphere was observed for complexes with 5-

pyrrolide substituents. The two THF ligands in Zr-

characterized by X-ray crystallography (Figure 1) and, with the

exception of Zr( PDP ) (THF) , show distorted octahedral

coordination environments similar to those reported previously

Ph 36

Mes

Ph 38

for Zr( PDP )

and Zr( PDP ) . One major dis-

tortion of the ZrN core, resulting in symmetry lowering from

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Figure 1. ORTEP representation of the molecular structures of Zr( PDP ) complexes with 30% probability ellipsoids. The structures on the

light-blue background are rotated 90° from the structures on the light-orange background. Only one of the two independent molecules in the

asymmetric unit of Zr( PDP^C)₂is shown. Hydrogen atoms were omitted for clarity.

6 5

local O to D symmetry, is enforced by the limited bite angle

of the PDP backbone clearly evidenced by contracted

ﬂattening (θ < 90°) and wagging distortions (θ < 180°),

as shown in Figure 2.

From the θ and θ angles summarized in Table 1, it is clear

N_p_y_r_r_o_l_i_d_e−Zr−N_p_y_r_r_o_l_i_d_eangles around 140° within each pincer

that the 5-pyrrolide substituents exert the most signiﬁcant

eﬀect on the extent of solid-state distortion. The sterically

demanding 2,4,6-trimethylphenyl (mesityl) substituents in

ligand. While the bond lengths for all Zr( PDP ) complexes

fall within a relatively narrow range (Zr−N_p_y_r_i_d_i_n_e= 2.28−2.31

^Å^;^Z^r⁻^Npyrrole = 2.14−2.19 Å), additional deviations from

Mes

Zr( PDP ) enforce a rigid structure with near-perfect

perfect D₂_dsymmetry for the Zr(PDP) unit are observed in

orthogonal arrangement of the ligands and strict trans

the solid-state structures of Zr( PDP ) but vary signiﬁcantly

coordination of the two pyridine rings (θ = 89.6°; θ

79.4°). The rigidity is further supported by intramolecular,

across the series of compounds. The most severe deviations

from D symmetry for each complex can be quantiﬁed by the

sandwich-type π-stacking interactions between the mesityl

substituents of one PDP ligand and the pyridine moiety of the

other (d_c_e_n_t_r_o_i_d₋_c_e_n_t_r_o_i_d≈ 3.6−3.7 Å). The strong steric

protection of the PDP π systems by the mesityl substituents

dihedral angle between the two planes deﬁned by the three

nitrogen atoms of each PDP pincer ligand, θ , and the

N_p_y_r_i_d_i_n_e−Zr−N_p_y_r_i_d_i_n_eangle, θ , with respective dihedral

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inﬂuence of the 3-pyrrolide substituents on the observed solid-

state distortions, with Zr( PDP ) , Zr( PDP ) , Zr-

C6F5

(

PDP ) , and Zr( PDP )₂all exhibiting very similar

molecular structures and distortions.

Despite their discussed structural diﬀerences in the solid

state, it is important to reemphasize that all complexes exhibit

time-averaged structures reﬂecting ideal D₂_dsymmetry on the

NMR time scale at room temperature in solution. Because all

photophysical, photochemical, and electrochemical experi-

ments described below were conducted in solution, distortions

in the solid-state structures are not necessarily representative of

the molecular structures under experimental conditions.

However, we propose that the solid-state structures are

indicative of the overall molecular ﬂexibility, which may

inﬂuence the accessibility of nonradiative relaxation pathways

through vibrational motion in solution and thereby may have

profound impacts on the photophysical properties of Zr-

(

PDP) photosensitizers.

Optical Properties of Zr( PDP ) Complexes. The

electronic absorption and emission spectra for all seven

Figure 2. Schematic representation of solid-state distortions for the

Zr( PDP ) complexes in a rigorously dried and deaerated

studied Zr( PDP ) complexes.

benzene solution at room temperature are shown in Figure 3,

and important parameters are summarized in Table 2 . For ease

Table 1. Comparison of the θ and θ Angles in the Solid

State from X-ray Crystallography

complex

dihedral angle θ (deg) N −Zr−N angle θ_w(deg)

Zr( PDP^C)₂

Zr( PDP^P^h)₂

Zr( PDP^H)₂

Zr( PDP^M^e)₂

86.2

88.3

88.4

89.6

72.9

76.1

175.0

171.8

169.1

173.1

179.4

153.8

151.8

Mes

Zr( PDP )

Measured as shown in Figure 2. Average value of two independent

molecules in the unit cell.

is also responsible for the remarkable stability of Zr-

Mes

(

PDP ) , making it the only Zr(PDP) derivative discussed

in this work that is stable under ambient conditions and does

not require rigorously dried solvents for solution experiments.

While π-stacking interactions are also observed in Zr-

(

PDP ) , the simple 5-Ph pyrrolide substituents can engage

in energetically favored, parallel-displaced π interactions,

resulting in a highly distorted molecular structure in the

solid state. Notably, one 5-Ph substituent of each PDP ligand

engages in π stacking with the pyridine ring of the other PDP

unit, while the second 5-Ph substituent can undergo π stacking

with one of the pyrrolide moieties because of the reduced

steric demand of the ligands.

For the remaining complexes, where no intramolecular π-

stacking eﬀects are possible, the presence of 5-pyrrolide

substituents still has important structural consequences. This is

highlighted through a comparison of Zr( PDP ) with

Zr( PDP ) . With completely exposed π systems, Zr-

PDP ) is open to strong intermolecular π stacking (Figure

S23 ), which results in signiﬁcant distortions of the individual

molecules in the crystal lattice (θ = 76.1°; θ = 151.8°).

Because of the steric demands of the 5-methyl substituents,

Figure 3. Absorption (top) and emission (bottom; excited at the

similar distortions are not favorable for Zr( PDP ) , and

lowest-energy absorption maxima) spectra of Zr( PDP ) (purple),

6 5

hence the complex shows a structure much closer to idealized

Zr( PDP )₂(blue), Zr( PDP ) (red), Zr( PDP ) (green),

Mes

D symmetry (θ = 88.4°; θ = 169.1°). A further comparison

Zr( PDP )₂(orange), Zr( PDP ) (yellow), and Zr( PDP )

with other 5-methyl-substituted complexes reveals the limited

(black) in a benzene solution at room temperature.

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Table 2. Comparison of the Optical Properties of Zr( PDP ) Recorded in a Benzene Solution at Room Temperature under

an Inert Atmosphere

complex

λ_m_a_x(abs) (nm cm⁻¹)

λ_m_a_x(em) (nm cm⁻¹

)

Stokes shift (cm⁻¹

)

τ (μs)

Φ_P_L

Zr( PDP^C)₂

Zr( PDP^P^h)₂

Zr( PDP^H)₂

Zr( PDP^M^e)₂

521/19190

532/18800

524/19080

531/18830

528/18940

547/18280

482/20750

430/23360

586/17070

591/16920

590/16950

587/17040

581/17210

629/15900

568/17610

570/17540

2129

1877

2135

1797

1728

2383

3141

5812

313

474

260

576

290

190

413

619

0.34

0.27

0.10

0.22

0.38

0.11

0.28

0.12

Mes

Zr( PDP )

Zr( PDP ) (THF)

Time-resolved emission data obtained in benzene at room temperature under an inert atmosphere (detection set to emission maximum, 516 nm

excitation). Measured under an inert atmosphere in benzene via the comparative method with Rhodamine 6G as the standard (λ = 488 nm; Φ_P_L

exc

0.94).

Figure 4. (Top) Frontier molecular orbitals of Zr( PDP ) obtained from DFT calculations (isovalue set to 0.05). (Bottom) Calculated frontier

molecular orbital energies (eV) for Zr( PDP ) . Computed metal contributions (%) are shown in gray to the left of the orbital energies.

of comparison, the absorption spectra were normalized using

the intensity of the strongest absorption band in the visible

region of each spectrum as the reference. Despite its more

limited optical window in the UV region, benzene was chosen

over solvents such as THF, acetonitrile, or chlorinated

hydrocarbons to prevent solvent coordination and allow a

LMCT character varies between 25 and 40% based on the

extent of d-orbital contributions to the acceptor orbitals.

Among the studied complexes, the two derivatives with aryl

Mes

substituents in the R position, Zr( PDP ) and Zr-

( PDP ) , exhibit the highest LMCT character at close to

40%, while the remaining complexes show values closer to

30%.

valid comparison between Zr( PDP ) and the remaining six-

coordinate species. Aside from Zr( PDP ) , the remaining

The subset comprised of the four complexes Zr( PDP )

Zr(PDP) complexes do not show any signiﬁcant solvato-

(R = C F , Ph, H, Me) allowed a more detailed analysis of the

2 6 5

chromism for absorption or emission.

isolated eﬀect of the R substituents on the absorption spectra

All seven Zr( PDP ) complexes show qualitatively similar

of Zr(PDP)₂complexes. Despite the markedly diﬀerent

electronic eﬀects of the substituents, from electron-with-

drawing (C F ) to electron-donating (Me), the absorption

spectra with strong absorption bands in the visible region of

the spectrum, which were assigned to transitions with mixed

singlet intraligand ( IL)/ LMCT character based on calcu-

lations employing time-dependent density functional theory

spectra remain remarkably similar, with absorption maxima

between 521 and 532 nm (Table 2) and a second band of

slightly lower intensity around 400 nm. Considering the more

prominent contributions of the pyrrolide π orbitals to the

donor [highest occupied molecular orbital (HOMO) and

HOMO−1] compared to the acceptor [degenerate lowest

unoccupied molecular orbital (LUMO) and LUMO+1]

orbitals for the lowest-energy transitions, this observation

seemed counterintuitive because a pronounced change in the

Ph 37

(

TD-DFT). As shown previously for Zr( PDP ) , the

donor orbitals for these transitions are exclusively ligand-

centered with predominant contributions from the electron-

rich pyrrolide π systems, while the acceptor orbitals show

mixed parentage with major contributions from the metal d_x_z

and d orbitals and the pyridine π systems (Figure 4). On the

basis of individual calculations for each complex, the amount of

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energy of the donor relative to the acceptor orbitals due to

substituent eﬀects should result in signiﬁcant shifts in the

transition energy and corresponding absorption maximum.

Further analysis of the frontier molecular orbitals obtained

from DFT calculations showed the expected impact of the R₂

substituents for the predominantly pyrrolide-centered HOMO

Table 2). On the basis of our detailed studies of the

photophysical properties and excited-state dynamics of Zr-

Mes

Ph 38

(

PDP ) , this long-lived emission in Zr(PDP) com-

plexes at and around room temperature can be assigned to

TADF because of a rapid thermal equilibrium between

energetically close-lying singlet and triplet excited states.

Under these conditions, the observed emission lifetime is a

function of multiple factors including the individual lifetimes of

the singlet and triplet excited states, S and T , and the energy

and HOMO−1. Using Zr( PDP ) as the electronically

neutral reference point, a stabilizing eﬀect is imparted by the

6 5

C F

C F groups in Zr( PDP )₂, while the Me substituents in

Zr( PDP ) result in a slight destabilization. However, these

gap between the two excited states, which is connected to the

relative ratio of the forward and reverse intersystem crossing

rate constants. Therefore, a straightforward correlation of the

lifetimes and quantum yields with other molecular properties is

diﬃcult and beyond the scope of this work.

eﬀects are almost completely compensated for by nearly

equivalent changes to the orbital energies of LUMO and

LUMO+1, resulting in very similar HOMO/LUMO gaps for

all Zr( PDP ) complexes (Figure 4). Despite the diﬀerent

contributions from the pyrrolide rings overall, the contribu-

tions from the carbon atoms at the 3 position of the

heterocycles are nearly identical between HOMO/HOMO−

An interesting case is presented by the closely related six-

and eight-coordinate complexes Zr( PDP )

and Zr-

(THF) . Despite the substantial diﬀerences in their

( PDP )

absorption spectra, the emission proﬁles of Zr( PDP )

and LUMO/LUMO+1. Because the electronic eﬀects of the

benzene and Zr( PDP ) (THF) in THF are nearly identical,

2 2

R₂substituents on the energy of the PDP π system need to be

mediated through the 3 position carrying the diﬀerent R₂

groups, equal contributions from the carbon atom in this

position to the frontier molecular orbitals provide a rationale

for the similar changes in both the HOMO/HOMO−1 and

LUMO/LUMO+1 energies and, consequently, the small eﬀect

of ligand substitution in these positions on the absorption

proﬁle.

resulting in a larger Stokes shift for the eight-coordinate

complex (Figure 5 ). On the one hand, this increase in the

Substitution in the R positions resulted in more signiﬁcant

changes to the absorption proﬁles that can be correlated with

the electronic properties of R . While the Ph substituents in

Zr( PDP ) are not perfectly coplanar with the pyrrolide

rings, partial conjugation extends the π system of the PDP

ligand. This results in a stabilization of LUMO/LUMO+1 and

a red-shifted absorption proﬁle compared to the Zr( PDP )

complexes with a low energy maximum at 547 nm. The bulky

mesityl substituents in Zr(^M^e^sPDP ) remove this conjugation

by enforcing a close to perfectly perpendicular arrangement

between R = Mes and the pyrrolide rings. As a consequence,

Mes

the absorption spectra of Zr( PDP ) and Zr( PDP ) are

almost identical, indicating similar inductive eﬀects of the

Figure 5. Absorption (solid line) and emission (dotted line; excited at

electron-donating methyl and electron-rich mesityl groups.

the lowest-energy absorption maxima) spectra of Zr( PDP )

(purple) in a benzene solution and Zr( PDP ) (THF) (dark

2 2

Finally, Zr( PDP ) possesses the most blue-shifted absorp-

tion with λ_m_a_x= 482 nm because of a lack of conjugation from

cyan) in a THF solution at room temperature.

either R or R substituents. The absorption maximum is

shifted even further toward higher energy upon coordination of

THF and the formation of Zr( PDP ) (THF) (λ = 430

Stokes shift may simply be the result of more pronounced

geometry changes between the ground and excited states for

max

nm). This additional blue shift is consistent with the LMCT

character of the absorption band because the added THF

ligands increase the electron density at the metal center,

resulting in less favorable charge transfer to zirconium.

Excitation with UV or visible light with wavelengths shorter

Zr( PDP ) (THF) , leading to increased energy loss through

internal conversion. Potential ligand dissociation and equilibria

between diﬀerent Zr( PDP ) (THF) (n = 0−2) species in

solution could amplify these processes. On the other hand, the

larger energy gap between absorption and emission for the

eight-coordinate complex could also indicate changes in the

primary nature of the emission between Zr( PDP ) and

Zr( PDP ) (THF) . It is reasonable to assume that the

than 600 nm [shorter than 550 nm for Zr( PDP ) ] in

solution resulted in strong photoluminescence for all

complexes. Consistent with the trend observed in the

absorption spectra, the four methyl-substituted complexes

Mes

Zr( PDP ) and Zr( PDP ) show very similar emission

primary emission mechanism in the six-coordinate complex

proﬁles with maxima in a narrow range of 581−591 nm, while

Zr( PDP ) is TADF, but the situation is less clear for

Zr( PDP ) and Zr( PDP ) display red- and blue-shifted

Zr( PDP ) (THF) . A plausible hypothesis for the observed

2 2

emission at 629 and 568 nm, respectively. Room-temperature

changes in the photophysical properties of Zr-

photoluminescence quantum eﬃciencies were determined by

( PDP )

(THF) is direct emission from the lowest-energy

2 2

55,56

the comparative method

in deaerated benzene solutions

triplet state via phosphorescence. The diﬀerence in the Stokes

−

and span a range of Φ = 0.10−0.38 (Table 2). Long emission

shift of 2671 cm between the six- and eight-coordinate

lifetimes of hundreds of microseconds were observed for all

complexes is on the same order of magnitude as the singlet−

−

compounds under the same conditions (τ_P_L= 190−576 μs;

triplet energy gap, ΔE_S₁₋_T₁= 1650 cm , observed

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Forum Article

experimentally for Zr(^M^e^sPDP ) . This explanation is also

consistent with the lifetime and quantum yield data. Zr-

(

PDP ) (THF) shows a signiﬁcantly longer emission

2 2

lifetime (619 vs 413 μs) with a lower photoluminescence

quantum yield (0.12 vs 0.28), indicating very similar rates of

−

nonradiative excited-state deactivation (1422 vs 1744 s ) but

a change by a factor of more than 3 in the radiative decay rates

−

(

194 vs 678 s ). Experimental validation of this hypothesis

through temperature-dependent emission studies is beyond the

scope of this work but will be attempted in future studies.

Beyond the analysis of electronic substituent eﬀects on the

optical properties, we were interested in investigating potential

correlations between the structural diﬀerences observed in the

solid state and the solution luminescence properties of

Zr( PDP ) . On the basis of the optical properties

summarized in Table 2, neither the photoluminescence

quantum yield nor the emission lifetime shows a clear

correlation with the observed structural distortions. This is

not surprising given that, for emission via TADF, both

parameters depend on a complex interplay of fundamental

processes including internal conversion, (reverse) intersystem

crossing, and nonradiative relaxation of the multiple excited

states involved. However, there is a clear correlation between

the observed Stokes shift between absorption and emission

maxima and the extent of molecular distortion in the solid

state. The Zr( PDP ) derivatives that experience the

smallest distortions in their solid-state structures exhibit the

smallest Stokes shifts (Figure 6). This is consistent with a

smaller structural reorganization for more rigid Zr(PDP)₂

complexes after photoexcitation and a reduced energy loss

through vibrational relaxation in the excited state. One notable

outlier is Zr( PDP ) , for which the Stokes shift is

signiﬁcantly smaller than expected based on its highly distorted

solid-state structure. This can most likely be attributed to a

weakening of the intramolecular π-stacking interactions in

aromatic solvents, which may result in a more symmetric

structure in solution compared to the solid state.

Figure 6. Correlation between θ and the Stokes shifts (top) and θ_w

and the Stokes shifts (bottom). For reasons discussed in the main

Electrochemistry and Excited-State Redox Potentials.

Cyclic voltammograms (CVs) were recorded in solutions

containing 0.1 M [N(n-Bu) ]PF as the supporting electrolyte

text, Zr PDP was excluded from the ﬁt.

by using a glassy-carbon working electrode. Ferrocene was

used as an internal standard, and all potentials are references

against the ferrocenium/ferrocene (Fc /Fc) couple. Figure 7

presence of electron-poor pentaﬂuorophenyl substituents that

are not stable under the highly reducing conditions. The

reduction potentials within the four-membered series Zr-

shows the CVs of the seven six-coordinate complexes

( PDP ) are strongly inﬂuenced by the nature of the R

Zr( PDP ) and Zr( PDP ) (THF) . The extracted redox

substituents with more positive redox potentials for the

electron-withdrawing groups (C F and Ph) and more negative

potentials are summarized in Table 3. To allow for solvent

stability under highly reducing conditions, CVs were recorded

in a THF solution. As the only exception, the CV of

potentials for the electron-donating Me substituents in

Zr( PDP ) . Over the complete series of Zr( PDP ) ,

Zr( PDP ) was recorded in 1,2-diﬂuorobenzene to avoid

the potentials for the ﬁrst reduction cover a range of nearly 600

solvent coordination and maintain the six-coordinate species in

solution. To establish comparability between the potentials

recorded in THF and 1,2-diﬂuorobenzene, the redox proper-

mV, highlighting the tunability of the electrochemical proper-

ties by means of the R substituents. In a similar fashion, the

methyl substituents on the R aryl substituents in Zr-

Mes

ties for Zr( PDP ) were measured in both solvents and

(

PDP ) shift the reduction to more negative potentials

show excellent agreement between experiments. Unfortunately,

compared to Zr( PDP ) . The observation that both R and

the smaller solvent window for 1,2-diﬂuorobenzene prevented

R₂substituents have an eﬀect on the ground-state redox

properties is consistent with the composition of the LUMO of

detection of the third reduction event for Zr( PDP )

reported previously and any potential reductions beyond

the Zr(PDP) motif, which shows signiﬁcant contributions of

−

3.0 V for Zr( PDP ) .

the carbon atoms at the 3 and 5 positions of the pyrrolide

moieties (Figure 4).

6 5

C F

With the exception of Zr( PDP )₂, all complexes show at

least two fully reversible redox events corresponding to two

consecutive single-electron reductions. The absence of a

second reversible reduction and the quasi-reversible nature of

For complexes with the lowest second reduction potentials,

Zr( PDP ) and Zr( PDP ) , a third reversible reduction

event could be detected at potentials beyond −3.1 V in THF.

The remarkable range of accessible redox states for [Zr-

6 5

C F

even the ﬁrst reduction in Zr( PDP )₂are likely due to the

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Forum Article

With ground-state reduction potentials, E₁_/₂(0/1−), and

emission energies, E , in hand, estimates for the potentials of

single-electron reduction of Zr( PDP ) complexes in the

excited state, E₁_/₂(0*/1−), were calculated as E (0*/1−) =

1/2

E₁_/₂(0/1−) + E . One caveat of this approach is

0,0

determination of the excited-state energy, E , from λ of

max

the emission band: Because the emission in Zr(PDP)₂

complexes is due to TADF rather than phosphorescence, the

observed emission spectrum at room temperature reﬂects an

approximation of the S₁excited-state energy. However,

excited-state electron transfer is more likely to occur from

the long-lived T state due to its higher thermal population

established via rapid S /T equilibrium following photo-

Mes

excitation. For Zr( PDP ) , the only complex for which

the triplet energy was determined via the detection of a

phosphorescence band by low-temperature emission spectros-

copy, this distinction amounts to a diﬀerence in E₀_,₀and the

resulting excited-state potential of approximately 0.2 eV.

Nevertheless, the estimated excited-state potentials using the

room temperature TADF emission maximum are useful

indicators for the feasibility of photoinduced single-electron

transfer during photocatalysis, in our experience. For example,

Mes

the excited state of Zr( PDP ) is reductively quenched by

Figure 7. CVs of Zr(_P_D_P_C6

₍_b_l_u_e₎_,_Z_r₍MePDP ) (red),

1,3-dimethyl-2-phenyl-2,3-dihydro-1H-7-methoxybenzo[d]-

)

MeO

Zr( ^P^D^P⁾2 (green), Zr( PDPMe₎(orange), Zr( PDP )

Mes

imidazole ( BIH), which possesses a reported redox

potential of −0.22 V. Using the phosphorescence energy of

(

yellow), Zr( PDP ) (black), and Zr( PDP ) (THF) (dark

Mes

Zr( PDP ) , the excited-state redox potential of the

cyan) in THF and Zr( PDP ) (purple) in 1,2-diﬂuorobenzene at

room temperature (scan rate 200 mV s ; 0.1 M [N(n-Bu) ]PF ;

glassy-carbon working electrode).

−

complex can be estimated as −0.31 V, which indicates that

MeO

excited-state reduction by

BIH is thermodynamically

disfavored. In contrast, using the TADF emission maximum,

a potential of −0.12 V is obtained, which suggests favorable

excited-state electron transfer in agreement with the exper-

imental data. Bearing this in mind, the E₀_,₀values reported in

Table 4 were derived from λ_m_a_xof the room-temperature

emission spectra.

Table 3. Redox Potentials for Zr( PDP ) and

Zr( PDP ) (THF)

E₁_/₂(0/1−)

E₁_/₂(1−/2−)

E₁_/₂(2−/3−)

complex

(V)

Zr( _P_D_P_C6 )₂

−1.85

−2.63

Zr( PDP )₂

−2.16

−3.22

(

−2.14 )

(−2.57 )

Table 4. Excited-State Potentials for Zr( PDP )

Zr( PDP^H)₂

−2.21

−2.42

−2.25

−2.09

−2.24

−2.45

−2.70

−2.85

−2.95

−2.67

−2.56

^E1/2(0/1−) (V, vs

^E1/2(0*/1−) (V, vs

+/0

Zr( PDP^M^e)₂

0,0

+/0

complex

)

(eV)

Fc )

Mes

Zr( PDP )

Zr( _P_D_P_C6

)

−1.85

−2.16

−2.21

−2.42

−2.25

−2.09

−2.24

2.12

2.10

2.11

2.13

1.97

2.18

+0.27

−0.06

−0.11

−0.31

−0.12

−0.06

Zr( PDP )

−3.13

Zr( PDP )

Zr( PDP^H)₂

Zr( PDP ) (THF)

−2.82

Zr( _P_D_PMe)₂

All potentials are referenced against the Fc /Fc redox couple.

Mes

Zr( PDP )

Quasi-reversible._eFrom ref 36. Potentials measured in 1,2-

Zr( PDP )

diﬂuorobenzene. The 1,2-diﬂuorobenzene solvent window is not

Zr( PDP )

wide enough to observe the third reduction event (E ≈ −3.0 V).

max

Approximated as the emission energy at room temperature.

From ref 38.

Calculated using E (0*/1−) = E (0/1−) + E . Quasi-reversible

1/2

0,0

redox event. From ref 36. From ref 38.

(

PDP) ] complexes under reducing conditions can be traced

to the redox activity of the PDP ligands. Each of the central

pyridine rings of the two PDP ligands can accept one electron,

As in the case of the optical properties and ground-state

redox potentials, the clearest trend for the excited-state redox

n−

as was demonstrated for the redox series [Cr( PDP ) ] (n

properties can be observed within the series Zr( PDP ) .

1, 2, or 3), in which all members contain a chromium(III)

Because the R substituents strongly aﬀect E (0/1−) but

1/2

Mes

ion. As reported previously, Zr( PDP ) exhibits unique

have no signiﬁcant inﬂuence on E , any changes to the

0,0

redox properties compared to the other complexes discussed in

this work with two additional redox events that can be

attributed to single-electron oxidations. As expected for a

zirconium(IV) species, these oxidations are exclusively ligand-

centered and are made possible by the increased steric

protection of the PDP π system provided by the mesityl

substituents.

ground-state redox properties directly propagate to the excited-

state redox potential E₁_/₂(0*/1−). This allows the straightfor-

ward tuning of E₁_/₂(0*/1−) over nearly 600 mV (−0.31 to

+0.27 V), with only minor changes in the absorption or

emission proﬁles. In contrast, changes in R aﬀect both the

redox properties and excited-state energies. For the complexes

studied in this work, the two eﬀects counterbalance almost

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Forum Article

completely because more negative reduction potentials are

associated with more blue-shifted emission bands, i.e., higher

^E⁰,0 values. Consequently, the excited-state potentials of the

Table 5. Stern−Volmer Constants and Quenching Rate

Constants for Zr( PDP )

Mes

(×10

E1/2(0*/1−) (V,

+/0

four complexes Zr( PDP ) , Zr( PDP ) , Zr( PDP ) ,

K_S_V₍_L_m_o_l−1

L mol_s−1

−1

complex

)

vs Fc )

and Zr( PDP ) cover a relatively narrow potential range of

Zr( _P_D_P_C6 )₂

446000 ± 5300

763000 ± 3800

158000 ± 1600

265000 ± 1500

31.0

23.5

11.5

4.9

+0.27

−0.06

−0.11

−0.31

only 60 mV (−0.06 to −0.12 V).

Zr( PDP )

Photoinduced Single-Electron Transfer. With excited-

Zr( PDP^H)₂

state potentials of +0.27 to −0.31 V, Zr( PDP ) photo-

Zr(^M^ePDP^M^e)₂

sensitizers should be considered to be mild oxidants in their

excited state and require relatively strong sacriﬁcial reductants

to participate in excited-state electron transfer. We previously

interested in testing the scope of potential substrates for

reductive photoredox catalysis. With few exceptions, previous

examples for photoredox catalysis using zirconium photo-

sensitizers relied on the use of activated alkyl halides or oleﬁns,

which are relatively easy targets for photochemical reduction.

In contrast, aryl halides are generally considered more

identiﬁed benzimidazolium hydride ( BIH) derivatives as

suitable electron donors that can quench the excited state of

Zr( PDP ) through photoinduced single-electron trans-

fer. One key piece of evidence supporting excited-state

electron transfer in these studies was the strong dependence of

the quenching rate constant on the redox potential of the BIH

derivative, which can be conveniently controlled through the

choice of substituent on the benzimidazolium fragment. With

a series of Zr( PDP ) complexes covering a range of

challenging substrates, with the reactivity decreasing from

aryl iodides to the more readily available aryl chlorides. As a

ﬁrst proof of principle, dehalogenation of methyl 4-

iodobenzoate was attempted using Zr( PDP ) as the

excited-state redox potentials in hand, we conducted the

reverse experiment, in which the potential of the sacriﬁcial

reductant was kept constant. To ensure suﬃcient reducing

(MeO) Ph

photocatalyst (10 mol %) and

BIH as the sacriﬁcial

reductant and hydrogen-atom donor. Following irradiation

with green light (light-emitting diode; λ_m_a_x= 520 nm) for 14 h,

the desired product, methyl benzoate, was obtained in good

yield with nearly quantitative conversion of the starting

material (Scheme 3). Independent control experiments in

the absence of light or photosensitizer resulted in quantitative

recovery of the starting material, highlighting the photo-

chemical nature of the reaction. When the electron-with-

drawing methyl carboxylate substituent was replaced with

bromide, dehalogenation of 4-bromo-1-iodobenzene using 1

power even for Zr( PDP ) , which exhibits the most

negative excited-state potential of −0.31 V, 1,3-dimethyl-2-

(

2,4,6-trimethoxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole

(

MeO) Ph

^B^I^H^;^Eox = −0.33 V) was identiﬁed as a suitable

quencher. The resulting Stern−Volmer plots for Zr( PDP )

R = Me, H, Ph, C F ) are shown in Figure 8 and exhibit a

2 6 5

(

MeO) Ph

equiv of

BIH furnished bromobenzene in good yield,

demonstrating excellent selectivity for reduction of the more

reactive aryl iodide. However, signiﬁcantly extended irradiation

times were required compared to those of dehalogenation of

the more activated methyl iodobenzoate. The addition of

(

MeO) Ph

excess

BIH resulted in the partial reduction of

bromobenzene, indicating that the photocatalyst is able to

engage aryl bromide substrates albeit with reduced eﬃciency.

Finally, the electron-donating methoxy group in 4-iodoanisole

resulted in a further reduction in reactivity and incomplete

conversion to anisole even after nearly 3 days of irradiation.

Nevertheless, the successful activation of an electron-rich

benzene derivative encouraged further experiments with more

challenging aryl chlorides.

Starting again with electron-poor substrates, methyl 4-

chlorobenzoate was readily reduced to methylbenzoate using

Figure 8. Stern−Volmer plots obtained via time-resolved emission

spectroscopy.

(MeO) Ph

Zr( PDP ) in combination with

BIH. Notably, no

signiﬁcant diﬀerence in the reaction times or yields was found

between the iodo- and chlorobenzoate substrates. The

dehalogenation reactivity with aryl chlorides was readily

extended to 4-chlorobenzonitrile as a second electron-poor

aryl halide but required a substantial increase in the irradiation

time to 24 h. Unfortunately, switching to less reactive aryl

chloride derivatives such as 1,2- or 1,3-dichlorobenzene

resulted in dramatically reduced conversions even after

extended irradiation times of up to 7 days. Despite the limited

substrate scope presented here, our studies provide the ﬁrst

example for photochemical activation of aryl chlorides using a

zirconium photosensitizer. Importantly, our photocatalytic

system is driven by relatively low-energy green light and

does not need electrochemical activation to access the highly

reducing potentials required. This demonstrates the potential

strictly linear dependence of the luminescence lifetime on the

(

MeO) Ph

concentration of

BIH. This behavior is indicative of a

diﬀusion-controlled dynamic quenching process, and the

corresponding Stern−Volmer constants, K , and quenching

rate constants, k , are summarized in Table 5. As expected for

quenching via electron transfer, the values for k are correlated

to the excited-state redox potentials of the Zr( PDP )

photosensitizer.

Following excited-state electron transfer with ⁽^M^e^O⁾³^P^hBIH,

the electron-rich photosensitizer Zr( PDP ) produces the

most powerful ground-state reductant of any Zr(PDP)₂

complex reported to date. Given the negative potential of

0/1−

E₁_/₂([Zr( PDP ) ] ) = −2.42 V vs Fc /Fc, we were

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Inorganic Chemistry

Forum Article

Scheme 3. Aryl Halide Dehalogenation Reactions Using Zr( PDP ) as the Photocatalyst

for further applications of Zr(PDP)₂chromophores in

photochemical transformations requiring strongly reducing

conditions.

concept for the further exploration of early-transition-metal

chromophores in challenging photocatalytic transformations

under mild conditions.

CONCLUSIONS

EXPERIMENTAL DETAILS

■

A family of seven photoluminescent bis(pyridinedipyrrolide)-

zirconium complexes has been synthesized and investigated by

X-ray crystallography, electronic absorption and emission

spectroscopy, and cyclic voltammetry. The substituents at

the 3 and 5 positions of the pyrrolide moieties have distinct

eﬀects on the photosensitizer characteristics of these Zr-

General Considerations. All air- and moisture-sensitive manip-

ulations were carried out using standard high-vacuum-line, Schlenk, or

cannula techniques or in an MBraun inert-atmosphere drybox

containing an atmosphere of puriﬁed nitrogen. Solvents for air- and

moisture-sensitive manipulations were dried and deoxygenated using

a Glass Contour Solvent Puriﬁcation System and stored over 4 Å

molecular sieves. Deuterated solvents for NMR spectroscopy were

purchased from Cambridge Isotope Laboratories and distilled from

sodium metal (C D ), 4 Å molecular sieves (CD CN), or CaH

(

PDP ) compounds. The steric proﬁle of the forward-facing

^R1 ^s^u^b^s^tⁱ^t^u^eⁿ^t^s^,^c^o^r^r^e^s^p^oⁿ^dⁱⁿ^g^t^o^t^h^e⁵^-^p^y^r^r^o^lⁱ^d^e^s^u^b^s^tⁱ^t^u^eⁿ^t^s^,

exerts a major inﬂuence on the molecular structure and the

overall stability of the complexes. This is reﬂected in the

exceptional air and water stabilities of Zr( PDP )

1,62

Ph 36

(CDCl ). Tetrabenzylzirconium,

Zr( PDP ) , and Zr-

Mes

Ph 38

(

PDP )

were prepared following literature procedures. All

Mes

remaining chemicals were purchased from commercial sources (Fisher

Scientiﬁc, VWR, and Sigma-Aldrich) and used without further

puriﬁcation. All solids were dried under high vacuum, and low-

boiling liquids were dried over CaH and vacuum-transferred into

oven-dried glassware in order to bring them into the glovebox. High-

compared to those of the remaining Zr( PDP ) complexes

and the facile formation of eight-coordinate Zr-

(

PDP ) (THF) from Zr( PDP ) , which is unique among

2 2 2

the reported Zr(PDP) complexes. Additionally, the absorp-

tion and emission proﬁles as well as the electrochemical

properties of the complexes are sensitive to the nature of the

^R1 ^s^u^b^s^tⁱ^t^u^eⁿ^t^s^,ⁱⁿ^dⁱ^c^a^tⁱⁿ^g^sⁱ^gⁿⁱ^ﬁ^c^aⁿ^t^e^l^e^c^t^r^oⁿⁱ^c^e^ﬀ^e^c^t^s^t^h^r^o^u^g^h

extension of the ligand π system by conjugation. In contrast,

the backward-facing R substituents at the 3 position of the

boiling liquid substrates were degassed, dried over CaH , and ﬁltered

through Celite in the glovebox.

Physical Measurements. H and C{ H} NMR spectra were

recorded on an Agilent 400 MHz spectrometer, a JEOL 400 MHz YH

spectrometer, or a Varian INOVA 600 MHz spectrometer. All

chemical shifts are reported relative to SiMe using H (residual)

pyrrolide units contribute exclusively through electronic eﬀects

and have negligible contributions to the structural properties of

the Zr(PDP) core. Most importantly, changes to the R

substituents substantially impact the electrochemical properties

of the complexes, while the optical properties remain almost

unchanged. The small inﬂuence on the absorption and

emission proﬁles can be rationalized through similar eﬀects

chemical shifts of the solvent as a secondary standard. Electronic

spectra were recorded using a Shimadzu UV-1800 spectrophotometer

in gastight quartz cuvettes with 10 mm path length and ﬁtted with J-

Young valves. Emission spectra were obtained in 10-mm-path-length

gastight quartz cuvettes with J-Young valves using a Shimadzu RF-

5301 PC spectroﬂuorophotometer. Photoluminescent decay data

were collected using a Horiba Jobin-Yvon Fluorolog-3 spectroﬂuor-

ometer with a single-photon-counting module in multichannel scaler

mode and a 516 nm SpectraLED pulsed excitation source. Lifetimes

were determined through exponential ﬁtting using the provided decay

analysis software package, DAS v6.1. Cyclic voltammetry measure-

ments were conducted under a nitrogen atmosphere inside a MBraun

drybox using a Gamry Interface 1000 electrochemical workstation in a

single-compartment cell using 1 mM sample solutions with 0.1 M

tetrabutylammonium hexaﬂuorophosphate as the supporting electro-

lyte. A three-electrode setup was employed with glassy carbon

electrode as the working electrode, a platinum sheet as the counter

electrode, and a silver wire as the quasi-reference electrode. Ferrocene

of the R substituents on the energies of the ﬁlled and empty

frontier molecular orbitals. As a consequence, manipulation of

the R substituents allows convenient tuning of the ground-

and excited-state redox properties of Zr( PDP ) complexes

independent of the absorption and emission energies. All seven

complexes produce powerful ground-state reductants following

excited-state electron transfer with BIH derivatives through

reductive quenching. The most reducing complex, Zr-

(

PDP ) , was evaluated for its photochemical reactivity

toward aryl iodides and more challenging aryl chloride

substrates. The observed dehalogenation reactions represent

the ﬁrst examples for photochemical aryl halide activation with

group 4 photosensitizers and provide suﬃcient proof of

(

Fc) was added as an internal standard after completion of the

measurements, and all potentials were referenced versus the Fc /Fc

couple. High-resolution mass spectrometry (HRMS) spectra were

obtained on a Thermo Finnigan Linear Trapping Quadrupole mass

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Forum Article

spectrometer. Elemental analyses were performed at Robertson

Microlit Laboratories, Inc., in Ledgewood, NJ.

PyH). ¹³C NMR (101 MHz, CDCl ): δ 149.76, 137.32, 131.56,

119.71, 115.34, 110.45, 107.53. HRMS (ESI). Calcd for C H N

,6-Bis(3,5-diphenyl-1H-pyrrol-2-yl)pyridine (H₂PDP ). The

([M + H] ): m/z 210.10257. Found: m/z 210.10249.

synthesis was carried out according to a modiﬁed literature

2,6-Bis(3,5-dimethyl-1H-pyrrol-2-yl)pyridine (H₂^M^ePDP^M^e). The

synthesis was carried out according to a modiﬁed literature

36,52

procedure.

.00 equiv), 1,3-diphenylprop-2-en-1-one (6.17 g, 29.60 mmol, 2.00

equiv), and 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride

2.395 g, 8.88 mmol, 0.6 equiv) were mixed in a 250 mL Schlenk ﬂask

2,6-Pyridinedicarboxaldehyde (2.00 g, 14.80 mmol,

procedure. Bis(aminomethyl)pyridine (2.44 g, 17.79 mmol, 1.00

equiv), acetylacetone (3.65 g, 36.46 mmol, 2.05 equiv), and p-

toluenesulfonic acid hydrate (2.14 g, 12.45 mmol, 0.70 equiv) were

added to a 250 mL ﬂask charged with 150 mL of xylene. The reaction

ﬂask was ﬁtted with a Dean−Stark trap and a reﬂux condenser. The

mixture was reﬂuxed for 4 days. After it was cooled, the crude material

was passed through a column of silica. Removal of xylene and

(

under an argon atmosphere. Absolute ethanol was added, and the

mixture was heated to reﬂux. A solution of sodium tert-butoxide (853

mg, 8.88 mmol, 0.6 equiv) in ethanol was added via a syringe, and

heating was continued for 24 h. The reaction was cooled to room

temperature, and ammonium acetate (6.85 g, 88.81 mmol, 6.00

equiv) was added to the mixture. The reaction mixture was heated to

reﬂux open to air for 24 h. The solid precipitate was collected via

ﬁltration, washed with cold ethanol, and dried under high vacuum to

yield the desired product as a pale-yellow solid. Yield: 6.018 g, 79%.

recrystallization from ethyl acetate and hexane furnished H₂PDP

as a yellow solid. Yield: 527 mg, 11%; one crop. Mp: 203−205 °C. H

NMR (400 MHz, C D ): δ 10.19 (s, 2H, NH), 7.29 (dd, J = 8.8 and

7.2 Hz, 1H, p-PyH), 7.18 (d, J = 8.0 Hz, 2H, m-PyH), 5.75 (d, J = 2.8

Hz, 2H, PyH), 2.34 (s, 6H, CH ), 1.69 (s, 6H, CH ). C NMR (101

Mp: 231−233 °C. H NMR (400 MHz, C D ): δ 9.50 (s, 2H, NH),

MHz, C D ): δ 151.90, 137.57, 129.44, 125.90, 120.63, 114.81,

6 6

.59 (d, J = 7.2 Hz, 4H, PhH), 7.32 (d, J = 7.6 Hz, 4H, PhH), 7.24 (t,

112.35, 14.57 (d, J = 2.6 Hz), 12.24. HRMS (ESI). Calcd for

J = 7.6 Hz, 4H, PhH), 7.18−7.10 (m, 8H, PyH and PhH), 7.05 (t, J =

C H N ([M + H] ): m/z 266.16517. Found: m/z 266.16491.

17 20 3

.2 Hz, 2H, PhH), 6.72 (d, J = 8.0 Hz, 1H, p-PyH), 6.68 (d, J = 2.4

2,6-Bis(5-methyl-3-pentaﬂuorophenyl-1H-pyrrol-2-yl)pyridine

6 5

Me F

PDP^C). The synthesis was carried out according to a modiﬁed

36,52

Hz, 2H, PyH). C NMR (101 MHz, C D ): δ 150.83, 137.75,

literature procedure.

36.65, 133.39, 132.41, 129.76, 129.20, 128.92, 128.15 127.55,

27.12, 126.88, 124.48, 117.89, 110.89. HRMS (ESI). Calcd for

A 250 mL Schlenk ﬂask was charged with

2,6-pyridinedicarboxaldehyde (1.000 g, 7.400 mmol, 1.00 equiv), (E)-

4-pentaﬂuorophenylbut-3-en-2-one (3.530 g, 14.95 mmol, 2.02

equiv), and 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride

(1.198 g, 4.44 mmol, 0.60 equiv) under an argon atmosphere.

Absolute ethanol was added followed by a solution of sodium tert-

butoxide (0.427 g, 4.44 mmol, 0.60 equiv) in ethanol via a syringe.

The mixture was reﬂuxed for 24 h. Following cooling to room

temperature and opening to air, ammonium acetate (3.423 g, 44.4

mmol, 6.00 equiv) was added to the mixture. The reaction mixture

was again reﬂuxed for 24 h. The resulting orange-red solution was

reduced to a thick goo via rotary evaporation. This was then dissolved

in ethyl acetate and washed twice with a saturated sodium bicarbonate

C H N ([M + H] ): m/z 514.22777. Found: m/z 514.22885.

,6-Bis(5-methyl-1H-pyrrol-2-yl)pyridine (H₂PDP ). The syn-

thesis was carried out according to a modiﬁed literature

36,52

procedure.

2,6-Pyridinedicarboxaldehyde (2.00 g, 14.80 mmol,

.00 equiv), methyl vinyl ketone (2.59 g, 37.00 mmol, 2.50 equiv),

and 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride (2.395

g, 8.88 mmol, 0.6 equiv) were mixed in a 250 mL Schlenk ﬂask under

an argon atmosphere. Absolute ethanol was added, and the mixture

was heated to reﬂux. A solution of sodium tert-butoxide (853 mg, 8.88

mmol, 0.6 equiv) in ethanol was added via a syringe, and heating was

continued for 24 h. The reaction was cooled to room temperature,

and ammonium acetate (11.41 g, 148.02 mmol, 10.00 equiv) was

added to the mixture. The reaction mixture was heated to reﬂux open

to air. After 24 h, the solvent was removed. The resultant was diluted

solution, twice with H

acetate and puriﬁcation by silica gel column chromatography using

O, and twice with brine. Removal of ethyl

6 5

C F

hexane/ethyl acetate (8:1) as the eluent furnished H

orange-yellow solid. Yield: 1.611 g, 76%. Mp: 184−186 °C. H NMR

(400 MHz, C ): δ 9.02 (s, 2H, NH), 6.88 (t, J = 7.6 Hz, 1H, PyH),

6.66 (d, J = 8.0 Hz, 2H, PyH), 5.97 (d, J = 2.4 Hz, 2H, PyH), 1.83 (s,

PDP

as an

with H O, and the crude product was extracted with dichloromethane

(

DCM) three times. The combined organic layers were washed with

6 6

brine and dried over Na SO . Removal of DCM and puriﬁcation by

silica gel column chromatography using hexane/ethyl acetate (20:1)

6H, CH

). C NMR (101 MHz, C D ): δ 150.3, 144.9 (dm, J = 245

as the eluent furnished H PDP as a yellow solid. Yield: 1.34 g,

Hz), 140.4 (dm, J = 252 Hz), 138.2 (dm, J = 251 Hz), 137.6, 130.4,

8%. Mp: 211−213 °C. H NMR (400 MHz, C D ): δ 10.65 (s, 2H,

128.5, 116.2, 112.7 (td, J = 18.4 and 3.8 Hz), 111.7, 107.4, 12.4.

NH), 7.15−7.05 (m, 3H, p-PyH and m-PyH), 6.72 (t, J = 3.2 Hz, 2H,

NMR (376 MHz, C ): δ −140.1 (dd, J = 23.7 and 7.9 Hz), −156.7

6 6

PyH), 5.88 (m, 2H, PhH), 1.64 (s, 6H, CH ). C NMR (101 MHz,

(t, J = 21.8 Hz), −162.6 (td, J = 23.7 and 8.3 Hz). HRMS (ESI).

+ +

C D ): δ 151.27, 138.17, 131.50, 129.65, 114.11, 109.84, 109.21,

Calcd for C27

570.10205.

6 5

C F

H F N ([M + H] ): m/z 570.10226. Found: m/z

14 10 3

2.22. HRMS (ESI). Calcd for C H N ([M + H] ): m/z

38.13387. Found: m/z 238.13378.

Zr( PDP

) . LiHMDS (181 mg, 1.08 mmol, 2.02 equiv) in THF

,6-Bis(1H-pyrrol-2-yl)pyridine (H PDP ). The synthesis was

was added slowly to a 20 mL vial charged with a solution of

36,52

6 5

C F

carried out according to a modiﬁed literature procedure.

2,6-

PDP (300 mg, 0.527 mmol, 1.00 equiv) in 3 mL of THF. The

Dibromopyridine (3.6 g, 15.20 mmol, 1.00 equiv), 1-(tert-

butoxycarbonyl)-2-pyrroleboronic acid (9.4 g, 44.53 mmol, 2.93

equiv), K CO (18.90 g, 136.77 mmol, 9.00 equiv), and Pd(PPh )

reaction was stirred for 2 h at room temperature to generate a

luminescent dark-green-yellow solution of the deprotonated ligand. A

suspension of ZrCl (64 mg, 0.274 mmol, 0.52 equiv) in 3 mL of THF

3 4

(

878 mg, 0.760 mmol, 0.05 equiv) were placed in a 250 mL ﬂask

was added in small portions to the deprotonated ligand over 20 min.

The reaction mixture was allowed to stir at room temperature, and the

color of the solution slowly changed to emissive red. After 2 days,

THF was removed in vacuo and the residue triturated with diethyl

under an argon atmosphere. Then, 80 mL of a deaerated toluene/

ethanol/H O (9:3:4) solution was added via cannula transfer. The

reaction mixture was heated at 90 °C under argon for 24 h. After

cooling to room temperature, the solvent was removed. The residue

was extracted with ethyl acetate and then passed through silica gel to

remove boronic acid and palladium compounds. After removal of the

solvent, the residue was treated with ethylene glycol (50 mL) and

heated at 160 °C for 1 h under an argon atmosphere. After cooling to

room temperature, the mixture was extracted with CHCl . The

organic layer was washed with H O and brine and then dried over

Na SO . Removal of CHCl and recrystallization from CHCl and

hexane furnished H PDP as a green-yellow solid. Yield: 1.922 g,

ether (Et O) three times. The solid residue was then dissolved in a

minimum amount of ether and ﬁltered through Celite. The crude

product was puriﬁed by recrystallization [pentane/ether (2:1)] at −35

°C. Yield: 207 mg, 63%. H NMR (400 MHz, C

): δ 6.73 (t, J = 8.0

Hz, 2H, PyH), 6.41 (d, J = 7.6 Hz, 4H, PyH), 5.70 (s, 4H, PyH), 2.08

(s, 12H, CH ). C NMR (100 MHz, C D ): δ 154.2, 144.6 (dm, J =

246 Hz), 143.4, 141.6, 140.7 (dm, J = 252 Hz), 138.2 (dm, J = 247

Hz), 137.2, 114.5, 112.5, 111.5, 111.3 (td, J = 18.7 and 3.5 Hz), 14.3.

F NMR (376 MHz, C D ): δ −139.6 (dd, J = 24.1 and 7.9 Hz),

6 6

0%; one crop. Mp: 189−191 °C. H NMR (400 MHz, CDCl ): δ

.65 (s, 2H, NH), 7.61 (t, J = 8.0 Hz, 1H, PyH), 7.34 (d, J = 8.0 Hz,

H, PyH), 6.90 (m, 2H, PyH), 6.74 (m, 2H, PyH), 6.31 (m, 2H,

−155.2 (t, J = 21.4 Hz), −162.0 (td, J = 23.7 and 7.9 Hz). Anal. Calcd

for C H F N Zr: C, 52.90; H, 1.81; N, 6.85. Found: C, 52.79; H,

22 20

1.71; N, 6.62. Single crystals suitable for X-ray crystallographic

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Forum Article

analysis were grown from a pentane/Et O (2:1) solution of

generate a luminescent dark-green-orange solution of the deproto-

Zr( PDP^C) at −35 °C.

nated ligand. A solution of ZrCl (241 mg, 1.03 mmol, 0.52 equiv) in

Zr( PDP ) . A solution of tetrabenzylzirconium (109 mg, 0.239

3 mL of THF was added in small portions to the deprotonated ligand

over 20 min. The reaction mixture was allowed to stir at room

temperature, and the color of the solution slowly changed to emissive

red. After 2 days, THF was removed in vacuo and triturated with

Et O three times. The solid residue was washed three times with Et O

mmol, 1.00 equiv) in 1 mL of toluene was added slowly to a 20 mL

vial charged with a solution of H PDP (100 mg, 0.478 mmol, 2.00

equiv) in 2 mL of benzene. The reaction was stirred for 10 min at

room temperature, and the desired product crystallized out with

stirring. The reaction was then ﬁltered, and the solid was washed three

times with benzene. The product was collected as a dark-orange

to remove any unreacted Li₂PDP and potential monoligated

zirconium species. The crude product was dried in vacuo, redissolved

in DCM, and ﬁltered to remove LiCl and potentially unreacted ZrCl₄.

microcrystalline solid. Yield: 110 mg, 91%. H NMR (400 MHz,

C D ): δ 6.83 (t, J = 8.0 Hz, 2H, p-PyH), 6.80 (d, J = 1.6 Hz, 4H,

Removal of the solvent in vacuo provided a red microcrystalline solid.

PyH), 6.54 (dd, J = 3.2 and 0.8 Hz, 4H, PyH), 6.44 (d, J = 8.0 Hz,

Yield: 494 mg, 81%. H NMR (400 MHz, CDCl

): δ 7.54 (t, J = 8.0

H, m-PyH), 6.06 (t, J = 2.8 Hz, 4H, PyH). C NMR (101 MHz,

Hz, 2H, p-PyH), 6.87 (d, J = 8.0 Hz, 4H, m-PyH), 5.52 (s, 4H, PyH),

C D ): δ 154.63, 142.64, 140.56, 131.44, 112.00, 111.55, 111.23.

2.27 (s, 12H, CH

CDCl ): δ 154.87, 141.77, 140.71, 135.86, 124.29, 113.22, 110.43,

14.49, 13.35. Anal. Calcd for C34 Zr: C, 66.09; H, 5.55; N,

), 1.86 (s, 12H, CH ). C NMR (101 MHz,

Anal. Calcd for C H N Zr: C, 61.75; H, 3.59; N, 16.62. Found: C,

1.59; H, 3.98; N, 16.33. Single crystals suitable for X-ray

H N

34 6

crystallographic analysis were grown from a saturated solution of

13.60. Found: C, 65.83; H, 5.47; N, 13.38. Single crystals suitable for

X-ray crystallographic analysis were grown from a dilute THF solution

Zr( PDP ) in C D at room temperature.

Zr( PDP ) . n-BuLi [0.499 mL (1.6 M solution in hexanes), 0.798

of Zr( PDP ) at −35 °C.

mmol, 2.05 equiv] was added slowly to a 20 mL vial charged with a

solution of H₂PDP (200 mg, 0.389 mmol, 1.00 equiv) in 5 mL of

THF. The reaction was stirred for 2 h at room temperature to

generate a luminescent dark-red-orange solution of the deprotonated

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02343.

■

ligand. A solution of ZrCl (47 mg, 0.202 mmol, 0.52 equiv) in 3 mL

of THF was added in small portions to the deprotonated ligand over

0 min. The reaction mixture was allowed to stir at room temperature,

Additional experimental procedures, spectroscopic and

crystallographic data, and computational details (PDF )

and the color of the solution slowly changed to emissive red. After 2

days, THF was removed in vacuo and triturated with Et O three

times. The solid residue was washed three times with benzene to

remove any unreacted Li₂PDP and potential monoligated

zirconium species. The crude product was dried in vacuo, redissolved

in DCM, and ﬁltered to remove LiCl and potentially unreacted ZrCl₄.

Removal of the solvent in vacuo followed by recrystallization from

Accession Codes

CCDC 2021670−2021675 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Et O at −35 °C provided a red microcrystalline solid. Yield: 176 mg,

1%. H NMR (600 MHz, CDCl ): δ 7.44−7.37 (m, 16H, PhH),

.35−7.28 (m, 12H, PhH), 6.97−6.89 (m, 12H, PhH), 6.81 (t, J = 7.8

Hz, 2H, p-PyH), 6.35 (d, J = 7.8 Hz, 4H, PyH), 6.09 (s, 4H, PyH).

AUTHOR INFORMATION

Corresponding Author

C NMR (151 MHz, CDCl ): δ 153.56, 145.66, 140.13, 136.82,

■

35.78, 135.49, 129.27, 129.26, 128.42, 128.24, 126.97, 126.85,

26.77, 113.73, 113.00. Anal. Calcd for C H N Zr·Et O: C, 78.82;

Carsten Milsmann − C. Eugene Bennett Department of

Chemistry, West Virginia University (WVU), Morgantown,

West Virginia 26506, United States; orcid.org/0000-0002-

9249-5199 ; Email: camilsmann@mail.wvu.edu

H, 5.09; N, 7.07. Found: C, 79.10; H, 5.16; N, 7.22. Single crystals

suitable for X-ray crystallographic analysis were grown from a

saturated Et O solution of Zr( PDP ) at −35 °C.

Zr( PDP ) . n-BuLi [2.23 mL (1.6 M solution in hexanes), 3.58

mmol, 2.02 equiv] was added slowly to a 20 mL vial charged with a

Authors

^s^o^l^u^tⁱ^oⁿ^o^f^H2 PDP (420 mg, 1.77 mmol, 1.00 equiv) in 5 mL of

THF. The reaction was stirred for 2 h at room temperature to

generate a luminescent dark-green-orange solution of the deproto-

Yu Zhang − C. Eugene Bennett Department of Chemistry, West

Virginia University (WVU), Morgantown, West Virginia

6506, United States; orcid.org/0000-0001-8948-1434

nated ligand. A solution of ZrCl (227 mg, 0.973 mmol, 0.55 equiv) in

Dylan C. Leary − C. Eugene Bennett Department of Chemistry,

West Virginia University (WVU), Morgantown, West Virginia

mL of THF was added in small portions to the deprotonated ligand

over 20 min. The reaction mixture was allowed to stir at room

temperature, and the color of the solution slowly changed to emissive

red. After 2 days, THF was removed in vacuo and triturated with

Et O three times. The solid residue was washed three times with Et O

to remove any unreacted Li₂PDP and potential monoligated

zirconium species. The crude product was dried in vacuo, redissolved

6506, United States; orcid.org/0000-0003-0588-8267

Anne M. Belldina − C. Eugene Bennett Department of

Chemistry, West Virginia University (WVU), Morgantown,

West Virginia 26506, United States

Jeﬀrey L. Petersen − C. Eugene Bennett Department of

Chemistry, West Virginia University (WVU), Morgantown,

West Virginia 26506, United States

ⁱⁿ^D^C^M^,^aⁿ^d^ﬁ^l^t^e^r^e^d^t^o^r^e^m^o^v^e^Lⁱ^C^l^aⁿ^d^p^o^t^eⁿ^tⁱ^a^l^l^y^uⁿ^r^e^a^c^t^e^d^Z^r^C^l4^.

Removal of the solvent in vacuo provided a red microcrystalline solid.

Yield: 283 mg, 57%. H NMR (400 MHz, C D ): δ 6.83 (t, J = 8.0

Hz, 2H, p-PyH), 6.53 (d, J = 3.2 Hz, 4H, PyH), 6.45 (d, J = 8.0 Hz,

H, m-PyH), 5.76 (dd, J = 3.2 and 0.8 Hz, 4H, PyH), 2.01 (s, 12H,

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02343

CH₃). C NMR (101 MHz, C D ): δ 154.72, 142.39, 141.93, 140.25,

11.83, 111.01, 110.36, 14.70. Anal. Calcd for C H N Zr· / DCM:

30 26 6 6

Notes

C, 62.91; H, 4.61; N, 14.59. Found: C, 63.17; H, 4.59; N, 14.36.

Single crystals suitable for X-ray crystallographic analysis were grown

from a pentane/THF (1:1) solution of Zr( PDP ) at −35 °C.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Zr( PDP ) . n-BuLi [2.54 mL (1.6 M solution in hexanes), 4.07

We thank WVU and the National Science Foundation (Grant

CHE-1752738) for ﬁnancial support. This work used X-ray

crystallography (Grant CHE-1336071) and NMR (Grant

mmol, 2.05 equiv] was added slowly to a 20 mL vial charged with a

solution of H₂PDP (527 mg, 1.99 mmol, 1.00 equiv) in 5 mL of

THF. The reaction was stirred for 2 h at room temperature to

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(21) Iwamura, M.; Takeuchi, S.; Tahara, T. Ultrafast Excited-State

Forum Article

CHE-1228336) equipment funded by the National Science

Foundation. The WVU High Performance Computing

facilities are funded by the National Science Foundation

EPSCoR Research Infrastructure Improvement Cooperative

Agreement 1003907, the State of West Virginia (WVEPSCoR

via the Higher Education Policy Commission), the WVU

Research Corporation, and faculty investments.

Dynamics of Copper(I) Complexes. Acc. Chem. Res. 2015, 48, 782−

91.

22) Lazorski, M. S.; Castellano, F. N. Advances in the Light

Conversion Properties of Cu(I)-Based Photosensitizers. Polyhedron

014, 82, 57−70.

23) Cunningham, C. T.; Cunningham, K. L. H.; Michalec, J. F.;

(

Mcmillin, D. R. Cooperative Substituent Effects on the Excited States

of Copper Phenanthrolines. Inorg. Chem. 1999, 38, 4388−4392.

24) Buldt, L. A.; Guo, X.; Vogel, R.; Prescimone, A.; Wenger, O. S.

REFERENCES

■

A Tris(Diisocyanide)Chromium(0) Complex Is a Luminescent

1) Zhang, S.; Yang, X.; Numata, Y.; Han, L. Highly Efficient Dye-

Sensitized Solar Cells: Progress and Future Challenges. Energy

Environ. Sci. 2013 , 6 , 1443 −1464.

Analog of Fe(2,2 ′-Bipyridine) . J. Am. Chem. Soc. 2017, 139,

985−992.

(25) Buldt, L. A.; Wenger, O. S. Chromium(0), Molybdenum(0),

(

2) Grat

C 2003, 4, 145−153.

3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-

Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663.

4) Nazeeruddin, M. K.; Baranoff, E.; Gratzel, M. Dye-Sensitized

Solar Cells: A Brief Overview. Sol. Energy 2011, 85, 1172−1178.

5) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer,

zel, M. Dye-Sensitized Solar Cells. J. Photochem. Photobiol.,

and Tungsten(0) Isocyanide Complexes as Luminophores and

Photosensitizers with Long-Lived Excited States. Angew. Chem., Int.

Ed. 2017, 56, 5676−5682.

(26) Otto, S.; Grabolle, M.; Forster, C.; Kreitner, C.; Resch-Genger,

U.; Heinze, K. [Cr(Ddpd) ] : A Molecular, Water-Soluble, Highly

NIR-Emissive Ruby Analogue. Angew. Chem., Int. Ed. 2015, 54,

T. The Triplet State of Organo-Transition Metal Compounds. Triplet

11572−11576.

Harvesting and Singlet Harvesting for Efficient OLEDs. Coord. Chem.

Rev. 2011, 255, 2622−2652.

(27) Wang, C.; Otto, S.; Dorn, M.; Kreidt, E.; Lebon, J.; Srsan, L.;

Di Martino-Fumo, P.; Gerhards, M.; Resch-Genger, U.; Seitz, M.;

Heinze, K. Deuterated Molecular Ruby with Record Luminescence

Quantum Yield. Angew. Chem., Int. Ed. 2018, 57, 1112−1116.

6) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu, X.

Recent Progress in Metal-Organic Complexes for Optoelectronic

(28) Treiling, S.; Wang, C.; Forster, C.; Reichenauer, F.; Kalmbach,

J.; Boden, P.; Harris, J. P.; Carrella, L. M.; Rentschler, E.; Resch-

Genger, U.; Reber, C.; Seitz, M.; Gerhards, M.; Heinze, K.

Luminescence and Light-Driven Energy and Electron Transfer from

an Exceptionally Long-Lived Excited State of a Non-Innocent

Chromium(III) Complex. Angew. Chem., Int. Ed. 2019, 58, 18075−

Applications. Chem. Soc. Rev. 2014, 43, 3259−3302.

(7) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light

Photoredox Catalysis with Transition Metal Complexes: Applications

in Organic Synthesis. Chem. Rev. 2013, 113, 5322 −5363.

(8) Arias-Rotondo, D. M.; McCusker, J. K. The Photophysics of

8085.

29) Fredin, L. A.; Pap

Sundstrom, V.; Persson, P. Exceptional Excited-State Lifetime of an

Photoredox Catalysis: A Roadmap for Catalyst Design. Chem. Soc.

Rev. 2016, 45, 5803−5820.

(

ai, M.; Rozsalyi, E.; Vanko, G.; Warnmark, K.;

́ ́ ́

(9) Glaser, F.; Wenger, O. S. Recent Progress in the Development of

Iron(II)- N -Heterocyclic Carbene Complex Explained. J. Phys. Chem.

Lett. 2014, 5, 2066−2071.

Transition-Metal Based Photoredox Catalysts. Coord. Chem. Rev.

020, 405, 213129.

10) Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis.

Chem. Rev. 2016, 116, 10075 −10166.

11) Xuan, J.; Xiao, W.-J. Visible-Light Photoredox Catalysis. Angew.

Chem., Int. Ed. 2012, 51, 6828−6838.

12) Angnes, R. A.; Li, Z.; Correia, C. R. D.; Hammond, G. B.

(

30) Chabera, P.; Kjaer, K. S.; Prakash, O.; Honarfar, A.; Liu, Y.;

Fredin, L. A.; Harlang, T. C. B.; Lidin, S.; Uhlig, J.; Sundstro

Lomoth, R.; Persson, P.; Warnmark, K. FeII Hexa N-Heterocyclic

m, V.;

(

Carbene Complex with a 528 Ps Metal-To-Ligand Charge-Transfer

Excited-State Lifetime. J. Phys. Chem. Lett. 2018, 9, 459−463.

Recent Synthetic Additions to the Visible Light Photoredox Catalysis

31) Kjær, K. S.; Styring, S.; Persson, P.; Honarfar, A.; Uhlig, J.;

Ericsson, T.; Bendix, J.; Bergquist, K.-E.; War

Yartsev, A.; Rosemann, N. W.; Gordivska, O.; Chab

Lindh, L.; Kaul, N.; Lomoth, R.; Fredin, L. A.; Haggstro

P.; Sundstrom, V. Luminescence and Reactivity of a Charge-Transfer

Excited Iron Complex with Nanosecond Lifetime. Science 2019, 363,

49−253.

32) Chab

Honarfar, A.; Essen

Handrup, K.; Ericson, F.; Tatsuno, H.; Morgan, K.; Schnadt, J.;

Haggstrom, L.; Ericsson, T.; Sobkowiak, A.; Lidin, S.; Huang, P.;

Styring, S.; Uhlig, J.; Bendix, J.; Lomoth, R.; Sundstrom, V.; Persson,

P.; Warnmark, K. A Low-Spin Fe(III) Complex with 100-Ps Ligand-

to-Metal Charge Transfer Photoluminescence. Nature 2017, 543,

95−699.

33) Ashford, D. L.; Glasson, C. R. K.; Norris, M. R.; Concepcion, J.

nmark, K.; Prakash, O.;

era, P.; Strand, D.;

m, L.; Huang,

Toolbox. Org. Biomol. Chem. 2015 , 13 , 9152 −9167.

(13) Zeitler, K. Photoredox Catalysis with Visible Light. Angew.

Chem., Int. Ed. 2009, 48, 9785−9789.

(14) Tucker, J. W.; Stephenson, C. R. J. Shining Light on

Photoredox Catalysis: Theory and Synthetic Applications. J. Org.

Chem. 2012, 77, 1617−1622.

(

era, P.; Liu, Y.; Prakash, O.; Thyrhaug, E.; Nahhas, A. El;

, S.; Fredin, L. A.; Harlang, T. C. B.; Kjær, K. S.;

(15) Larsen, C. B.; Wenger, O. S. Photoredox Catalysis with Metal

Complexes Made from Earth-Abundant Elements. Chem. - Eur. J.

018, 24, 2039−2058.

16) Nocera, D. G. Solar Fuels and Solar Chemicals Industry. Acc.

Chem. Res. 2017, 50, 616−619.

17) For recent reviews, see the special issue on Solar Fuels: Chem.

Soc. Rev. 2013, 2205−2472.

18) Monro, S.; Colon, K. L.; Yin, H.; Roque, J.; Konda, P.; Gujar,

(

S.; Thummel, R. P.; Lilge, L.; Cameron, C. G.; McFarland, S. A.

Transition Metal Complexes and Photodynamic Therapy from a

Tumor-Centered Approach: Challenges, Opportunities, and High-

lights from the Development of TLD1433. Chem. Rev. 2019, 119,

J.; Keinan, S.; Brennaman, M. K.; Templeton, J. L.; Meyer, T. J.

Controlling Ground and Excited State Properties through Ligand

Changes in Ruthenium Polypyridyl Complexes. Inorg. Chem. 2014,

3, 5637−5646.

19) Forster, C.; Heinze, K. Photophysics and Photochemistry with

97−828.

Earth-Abundant Metals-Fundamentals and Concepts. Chem. Soc. Rev.

020, 49, 1057−1070.

20) Mara, M. W.; Fransted, K. A.; Chen, L. X. Interplays of Excited

34) Motley, T. C.; Troian-Gautier, L.; Brennaman, M. K.; Meyer,

G. J. Excited-State Decay Pathways of Tris(Bidentate) Cyclo-

metalated Ruthenium(II) Compounds. Inorg. Chem. 2017, 56,

13579−13592.

(35) Zhang, X. Z.; Cheng, C. C.; Chih, Y. R.; Lin, Y. T.; Chen, H. Y.;

Chen, Y. J.; Endicott, J. F. Low-Temperature Spectra and Density

Functional Theory Modeling of Ru(II)-Bipyridine Complexes with

Cyclometalated Ancillary Ligands: The Excited State Spin-Orbit

State Structures and Dynamics in Copper(I) Diimine Complexes:

Implications and Perspectives. Coord. Chem. Rev. 2015, 282−283, 2−

18.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(54) Komine, N.; Buell, R. W.; Chen, C.-H.; Hui, A. K.; Pink, M.;

Forum Article

Coupling Origin of Variations in Emission Efficiencies. J. Phys. Chem.

A 2019, 123, 9431−9449.

Caulton, K. G. Probing the Steric and Electronic Characteristics of a

New Bis-Pyrrolide Pincer Ligand. Inorg. Chem. 2014, 53, 1361−1369.

(36) Zhang, Y.; Petersen, J. L.; Milsmann, C. A Luminescent

Zirconium(IV) Complex as a Molecular Photosensitizer for Visible

(55) Wurth, C.; Grabolle, M.; Pauli, J.; Spieles, M.; Resch-Genger,

Light Photoredox Catalysis. J. Am. Chem. Soc. 2016, 138, 13115−

U. Relative and Absolute Determination of Fluorescence Quantum

Yields of Transparent Samples. Nat. Protoc. 2013 , 8 , 1535 −1550.

(56) Brouwer, A. M. Standards for Photoluminescence Quantum

Yield Measurements in Solution (IUPAC Technical Report). Pure

Appl. Chem. 2011, 83, 2213−2228.

37) Zhang, Y.; Lee, T. S.; Petersen, J. L.; Milsmann, C. A Zirconium

3118.

Photosensitizer with a Long-Lived Excited State: Mechanistic Insight

into Photo-Induced Single Electron Transfer. J. Am. Chem. Soc. 2018,

(57) Gowda, A. S.; Petersen, J. L.; Milsmann, C. Redox Chemistry of

(

40, 5934−5947.

Bis(Pyrrolyl)Pyridine Chromium and Molybdenum Complexes: An

Experimental and Density Functional Theoretical Study. Inorg. Chem.

38) Zhang, Y.; Lee, T. S.; Favale, J. M.; Leary, D. C.; Petersen, J. L.;

Scholes, G. D.; Castellano, F. N.; Milsmann, C. Delayed Fluorescence

from a Zirconium(IV) Photosensitizer with Ligand-to-Metal Charge-

Transfer Excited States. Nat. Chem. 2020, 12, 345.

58) Zhu, X.-Q.; Zhang, M.-T.; Yu, A.; Wang, C.-H.; Cheng, J.-P.

018, 57, 1919−1934.

Hydride, Hydrogen Atom, Proton, and Electron Transfer Driving

Forces of Various Five-Membered Heterocyclic Organic Hydrides and

Their Reaction Intermediates in Acetonitrile. J. Am. Chem. Soc. 2008,

(39) Pfennig, B. W.; Thompson, M. E.; Bocarsly, A. B. A New Class

of Room Temperature Luminescent Organometallic Complexes:

Luminescence and Photophysical Properties of Permethylscandocene

Chloride in Fluid Solution. J. Am. Chem. Soc. 1989 , 111 , 8947 −8948.

(

30, 2501−2516.

59) Nguyen, J. D.; D ’Amato, E. M.; Narayanam, J. M. R.;

(40) Paulson, S.; Sullivan, B. P.; Caspar, J. V. Luminescent Ligand-

Stephenson, C. R. J. Engaging Unactivated Alkyl, Alkenyl and Aryl

Iodides in Visible-Light-Mediated Free Radical Reactions. Nat. Chem.

to-Metal Charge-Transfer Excited States Based on Pentamethylcyclo-

pentadienyl Complexes of Tantalum. J. Am. Chem. Soc. 1992, 114,

(

012, 4, 854.

60) Cowper, N. G. W.; Chernowsky, C. P.; Williams, O. P.;

905−6906.

41) Heinselman, K. S.; Hopkins, M. D. Luminescence Properties of

Wickens, Z. K. Potent Reductants via Electron-Primed Photoredox

Catalysis: Unlocking Aryl Chlorides for Radical Coupling. J. Am.

Chem. Soc. 2020, 142, 2093−2099.

d Metal-Imido Compounds. J. Am. Chem. Soc. 1995, 117, 12340−

2341.

42) Loukova, G. V.; Smirnov, V. A. Phosphorescent Ligand-to-

(61) Radlauer, M. R.; Agapie, T. Bimetallic Zirconium Amine

Metal Charge-Transfer Excited States in the Group IVB Metallocene

Bis(Phenolate) Polymerization Catalysts: Enhanced Activity and

Tacticity Control for Polyolefin Synthesis. Organometallics 2014, 33,

Triad. Chem. Phys. Lett. 2000, 329, 437 −442.

(43) Loukova, G. V.; Smirnov, V. A. Phosphorescence of Group IVB

62) Rong, Y.; Al-Harbi, A.; Parkin, G. Highly Variable Zr-CH -Ph

247−3250.

Metal Complexes. Russ. Chem. Bull. 2001, 50, 329−330.

(44) Loukova, G. V.; Vasiliev, V. P.; Smirnov, V. A. Intense

Bond Angles in Tetrabenzylzirconium: Analysis of Benzyl Ligand

Coordination Modes. Organometallics 2012, 31, 8208−8217.

Luminescence of the Group IVB Metal π -Complex in Solutions at

Room Temperature. Russ. Chem. Bull. 2007, 56, 181−182.

(45) Loukova, G. V.; Huhn, W.; Vasiliev, V. P.; Smirnov, V. A.

Ligand-to-Metal Charge Transfer Excited States with Unprecedented

Luminescence Yield in Fluid Solution. J. Phys. Chem. A 2007, 111,

46) Loukova, G. V.; Milov, A. A.; Vasiliev, V. P.; Minkin, V. I.

117−4121.

Nature of Ligand to Metal Charge Transfer States in the Metallocene

Series M(η :η -CpCMe CB H C) (M = Ti, Zr, Hf). Russ. Chem.

10 10

Bull. 2017, 66, 1325−1327.

(47) Loukova, G. V.; Milov, A. A.; Vasiliev, V. P.; Minkin, V. I.

Frontier Orbitals and Ligand-to-Metal Charge Transfer Electronic

Transitions in d -Metal Complexes. High Energy Chem. 2017, 51,

(

33−337.

48) Romain, C.; Choua, S.; Collin, J. P.; Heinrich, M.; Bailly, C.;

Karmazin-Brelot, L.; Bellemin-Laponnaz, S.; Dagorne, S. Redox and

Luminescent Properties of Robust and Air-Stable N-Heterocyclic

Carbene Group 4 Metal Complexes. Inorg. Chem. 2014, 53, 7371−

(

376.

49) Gazi, S.; Hung Ng, W. K.; Ganguly, R.; Putra Moeljadi, A. M.;

Hirao, H.; Soo, H. S. Selective Photocatalytic C-C Bond Cleavage

under Ambient Conditions with Earth Abundant Vanadium

Complexes. Chem. Sci. 2015 , 6 , 7130 −7142.

(50) Wenger, O. S. Photoactive Complexes with Earth-Abundant

Metals. J. Am. Chem. Soc. 2018, 140, 13522−13533.

(51) Hockin, B. M.; Li, C.; Robertson, N.; Zysman-Colman, E.

Photoredox Catalysts Based on Earth-Abundant Metal Complexes.

Catal. Sci. Technol. 2019, 9, 889−915.

(52) Hakey, B. M.; Darmon, J. M.; Zhang, Y.; Petersen, J. L.;

Milsmann, C. Synthesis and Electronic Structure of Neutral Square-

Planar High- Spin Iron(II) Complexes Supported by a Dianionic

Pincer Ligand. Inorg. Chem. 2019, 58, 1252−1266.

(53) Jones, R. A.; Karatza, M.; Voro, T. N.; Civeir, P. U.; Franck, A.;

Ozturk, O.; Seaman, J. P.; Whitmore, A. P.; Williamson, D. J.

Extended Heterocyclic Systems 1. The Synthesis and Characterisation

of Pyrrolylpyridines, Alternating Pyrrole: Pyridine Oligomers and

Polymers, and Related Systems. Tetrahedron 1996, 52, 8707−8724.