Thermoluminescent Antimony-Supported Copper-Iodo Cuboids: Approaching NIR Emission via High Crystallographic Symmetry

This is an open access article published under an ACS AuthorChoice License, which permits

copying and redistribution of the article or any adaptations for non-commercial purposes.

Article

pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX −XXX

Thermoluminescent Antimony-Supported Copper-Iodo Cuboids:

Approaching NIR Emission via High Crystallographic Symmetry

William V. Taylor, Claudina X. Cammack, Soﬁa A. Shubert, and Michael J. Rose*

Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States

S

* Supporting Information

ABSTRACT: We report the syntheses, structures, and

luminescence properties of a series of copper-iodo cuboids

supported by L-type antimony ligands. The cuboids are of

general formula [(SbR₃)₄Cu₄(I)₄] (1−4, 8), where SbR₃is a

series of homoleptic and heteroleptic stibines containing both

i

t

phenyl and a variety of alkyl substituents (R = Cy, Pr, Bu,

i

Ph); triphenyl, Pr₂Ph, and Me₂Ph stibines resulted in the

formation of dimers of type [(SbR₃)₄(Cu)₂(I)₂] (5−7). While

similar luminescent copper-halide cubes have been studied,

the corresponding “heavy-atom” congeners have not been

studied, and ligation of such heavy-atom moieties is often

associated with long-lived triplet states and low-energy

absorption and emission proﬁles. Overall, two obligate

parameters are found to imbue NIR emission: (i) short Cu−Cu bonds and (ii) high crystallographic symmetry; both of

these properties are found only in [(SbⁱPr₃)₄Cu₄(I)₄] (1, in I23; λ_em= 711 nm). The correlation between NIR emission and

high crystallographic symmetry (which intrinsically includes high molecular symmetry)−versus only molecular symmetry−is

conﬁrmed by the counterexample of the molecularly symmetric ^tBu-substituted cuboid [(Sb^tBu₃)₄Cu₄(I)₄] (3, λ_em= 588 nm, in

R-3), which crystallizes in the lower symmetry trigonal space group. Despite the indication that the stronger donor strength of

the Sb^tBu₃ligand should red-shift emission beyond that of the SbⁱPr₃-supported cuboid, the emission of 3 is limited to the

visible region. To further demonstrate the connection between structural parameters and emission intensity, X-ray structures for

1 and 3 were collected between 100 and 300 K. Lastly, DFT calculations for 1 on its singlet (S₀) and excited triplet state (T₁)

demonstrate two key factors necessary for low-energy NIR emission: (i) a signiﬁcant contraction of the interconnected Cu₄

intermetallic contacts [∼2.45 → 2.35 Å] and (ii) highly delocalized (and therefore low-energy) A₁symmetry HOMO/LUMO

orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu₄core precludes the

formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T₁, thereby inhibiting low-energy NIR emission.

new classes of bright, long-lived phosphors for biological and

energy applications. However, there is a need for fundamental

studies of transition metal-based NIR phosphorescent

complexes, clusters, and materials.

INTRODUCTION

■

Transition metal complexes and clusters with luminescent

properties have found utility in ﬁelds ranging from OLEDs,

bioimaging devices, and pressure-sensing devices (mechano-

chromism).¹⁻³In addition, the discovery of new molecules and

materials that achieve NIR emission is desirable for

applications such as deep-tissue bioimaging, photodynamic

therapy, and novel two-photon and triplet-transfer solar

technologies.⁴⁻⁶Long-lived NIR emission (phosphorescence)

is diﬃcult to achieve due to the closer proximity of lower-lying

triplet states to thermally excited ground states.^7,8Additionally,

the presence of high energy oscillators (such as O−H bonds)

causes vibrational quenching in NIR emitters and limits long-

lived emissions.^9,10While a number of examples of organic-

based dyes and sensors for NIR emission and sensing have

been reported,^11,12these are largely limited to short-lived

ﬂuorescence rather than longer-lived phosphorescence, which

has a higher possibility for persistent emission past initial

excitation.¹³The use of transition metalsand, in conjunction,

heavy atoms to enhance intersystem crossingcould aﬀord

In particular, copper-based complexes are intriguing for their

low cost and relative stability. Luminescent copper complexes

exist in the form of monomers, dimers, tetramers, and

polymeric speciesand their emission wavelengths span the

entire visible spectrum.^14,15An interesting subset of

luminescent copper-halide compounds are the cubes with

general structure Cu₄X₄L₄(where X = halide; L = pnictogen

ligand). These complexes exhibit a unique property known as

luminescent thermochromism, in which emission intensity and

energy ﬂuctuate as a function of temperature. First discovered

by Hardt,¹⁶luminescent thermochromism is classically

explained as being derived from two triplet excited states:

one originating at the center of the copper cluster (³CC) and

Received: January 23, 2019

A

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

one originating from charge transfer from the halide to the

ligand (³XLCT).¹⁷Typically, a low energy (LE) band is

observed at room temperature and corresponds to the CC

transition, whereas at low temperatures, this band is abolished

and the presence of a high energy (HE) band emerges,

corresponding to the XLCT process. Notably, neither band

(speciﬁcally, the LE band) has been reported to occur in the

near-infrared (NIR) region.

Among published thermochromic copper cubes, the Cu₄I₄

core remains constant for the most part, but the supporting

ligand can be modulated. Photoactive clusters have been

reported containing nitrogen, phosphorus, sulfur, and arsenic

ligands; however, not all have been investigated for their

luminescent properties.¹⁸⁻²¹Our research has focused on

utilizing antimony-based supporting ligands with the aim of

modulating structural parameters²²and red-shifting emission

and absorption features due to the so-called “heavy atom

eﬀect”.²³⁻²⁵Additionally, our previous communication²²

demonstrated that the relatively weak σ donor strength of Sb

ligands allows for long Cu−Sb bonds andas a resultshort

Cu···Cu contacts that are within 2× the van der Waals

radius.^26,27

Extensive literature on phosphine-supported Cu₄I₄cubes has

elucidated a number of structure−activity design principles. In

general, phosphine-supported cubes exhibit luminescent

properties at ambient temperatures, regardless (without

known exception) of the phosphine substituents.¹⁷Second,

the emissive properties of these cubes are dependent on

temperature−primarily via thermochromism; that is, two

competing emissive states (LE = 500−600 nm, HE = 400−

500 nm; see paragraphs above) can be proportionally accessed

depending on the temperature. Studies have shown that

thermochromic properties (i.e., the presence of the HE XLCT

band) are dependent on the identity and number of phosphine

substituents.

To determine the eﬀect of a “heavy atom” approach, we

synthesized and reported the luminescent properties of

antimony-supported cluster 1,²²namely Cu₄(I)₄(SbⁱPr₃)₄.

Cluster 1 exhibits three luminescence properties that

distinguish it from the majority of phosphine clusters: (i) no

HE emission band is observed (not thermochromic = desired

for pure LE emitters); (ii) the emission is shifted into the NIR

at 710 nm; and (iii) the NIR luminescence is only observed at

low temperatures. Regarding point (i), based on the discussion

above (phenyl versus alkyl substituents), the lack of a HE band

is due simply to the lack of phenyl substituents on antimony

(precludes XLCT). While the resolution to point (iii)

remained ambiguous (addressed herein, vide infra), the NIR

LE emission from 1 was ascribed to structural features as

follows:

than the average Cu···Cu distance found in phosphine-

supported cubes (∼2.85−3.15). At ﬁrst glance, these results

suggest that close intermetallic contacts are required for

shifting the emission energy to the NIR. In contrast, our own

data for 1 regarding the structure−emission correlation

(temperature dependence) suggests that the Cu···Cu distance

controls the emission intensity not the emission energy. Thus,

based on all of the information presented above, our research

goals were to (i) shift the emission proﬁle further into the NIR,

(ii) resolve the apparent paradox involving the correlation

between Cu···Cu contacts and emission energy/intensity, and

(iii) develop a fundamental understanding of the correlations

between ligand identity, cluster structure, and emission

properties.

It is known (and conﬁrmed in our work) that SbPh₃does

not form cube or cuboid structures, and thus further ligand

design and synthetic rationale were required to access the

cuboid motif. Therefore, to fully elucidate the structure−

activity relationships within this class of Sb₄Cu₄I₄clusters, it

was necessary to synthesize and employ a wide range of

antimony-based ligandsincluding those with and without

phenyl units, with larger and smaller cone angles, with stronger

or weaker donor strength, as well as with varying extents of

symmetry. In this work, we report the syntheses, structures,

and solid-state luminescence properties of a series of antimony-

supported copper cuboids (and dimers) based on homoleptic

i

t

and heteroleptic SbR₃ligands (where R = Me, Pr, Bu, Cy,

Ph). A thorough analysis of the eﬀect of ligand sterics, as well

as molecular and crystallographic symmetries, is presented, and

the experimental ﬁndings are supported by DFT calculations

regarding the enigmatic origin of low energy luminescence.

EXPERIMENTAL SECTION

■

Reagents and General Procedures. All reactions were

conducted under a dry dinitrogen atmosphere (Schlenk line, ligand

syntheses) or under an argon atmosphere in a drybox (metalations).

Solvents were HPLC-grade and puriﬁed over alumina using a Pure

Process Technology solvent puriﬁcation system. Deuterated solvents

were purchased from Cambridge Laboratories and used without

further puriﬁcation, and the freeze−pump−thaw technique was used

to degas deuterated solvents as necessary. The starting materials

trichloroantimony and triphenylantimony were purchased from Strem

Chemicals and used without further puriﬁcation. Magnesium turnings

(Acros), iodine crystals (Acros), 2-chloropropane (Sigma-Aldrich),

iodomethane (Oakwood Chemical), tert-butyl magnesium chloride

2.0 M in Et₂O (Sigma-Aldrich), cyclohexyl magnesium chloride 1.0 M

in 2-MeTHF (Alfa Aesar), copper iodide (Strem Chemicals), and

ﬂuorobenzene (FPh, Oakwood Chemical) were purchased and used

as received without further puriﬁcation.

Ligand Syntheses. Safety Precaution. Alkyl-substituted anti-

mony ligands are volatile and air-sensitive and can be pyrophoric, and

they must be handled with caution under inert atmosphere. The

corresponding metal complexes are also highly air-sensitive and

sometimes pyrophoric and must be handled with similar care.

Diisopropylphenylantimony (SbⁱPr₂Ph). Trichloroantimony

(5.64 g, 24.7 mmol) and triphenylantimony (4.36 g, 12.4 mmol)

were stirred together solvent-free for 3 h to aﬀord dichlorophenylanti-

mony (10 g, 37.06 mmol) in quantitative yield. Magnesium turnings

(2.70 g, 111 mmol) were activated with iodine (0.01 g, 0.04 mmol)

and heated for 30 min under vacuum. Dry Et₂O (125 mL) was added

to the ﬂask, and 1,2-dibromoethane (0.22 g, 1.16 mmol) was

subsequently added. Next, 2-chloropropane (7.27 g, 92.6 mmol) was

added to the ﬂask at 0 °C and the reaction was stirred overnight. The

solution was ﬁltered. The SbCl₂Ph was then dissolved in Et₂O (90

mL) and added dropwise to the ﬁltered Grignard solution, and the

reaction was reﬂuxed for 3 h. The solution was then quenched with

The close Cu···Cu contacts in the ground state of 1 gave rise

to a pseudometallic Cu₄tetrahedron at the core of the cuboid,

with a 2.76 Å distances between all copper ions (crystallo-

graphically deﬁned) at 100 K. Notably, this distance is less

than 2× the van der Waals radius of copper (∼2.80 Å). In this

report,²²crystal structure solutions for 1 at varying temper-

atures between 100 and 300 K revealed a distinct structure−

emission correlation between shorter Cu···Cu distances [low

T: 2.761(3) → 2.836(4) Å, high T] and increased

luminescence. Indeed, as the Cu···Cu distance crossed 2×

the van der Waals radius of copper (∼2.80 Å), the emission

intensity exponentially increased. It is also notable that the

Cu···Cu distance found in 1 (∼2.76 Å) is distinctly shorter

B

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

degassed water (50 mL). Under inert atmosphere, the organic layer

was separated and dried over sodium sulfate and then ﬁltered. The

solvent was removed in vacuo to aﬀord a colorless oil. A fractional

distillation was performed, which ﬁrst distilled trace amounts of SbⁱPr₃

(75 °C, 0.01 mmHg), followed by the desired product (120 °C, 0.01

leaving the desired product as a white solid (0.15 g, 0.07 mmol).

Yield: 36%. Crystals suitable for X-ray crystallography were obtained

via slow vapor diﬀusion of pentane into a DCM solution of the

1

product at −20 °C. H NMR: (CDCl₃): 1.27 (mult 18H, −CH₂),

1.73 (mult 12H, −CH₂), 1.95 (mult 3H, −CH). ¹³C NMR (CDCl₃):

δ = 26.9, 29.0, 29.2, 32.3. Anal. cald (% wt.) for Cu₄I₄Sb₄C₇₂H₁₃₂: C,

38.49; H, 5.92. Found: C, 38.37; H, 5.95.

1

mmHg) as a colorless oil (4.7 g, 16.48 mmol). Yield: 45%. H NMR

(CDCl₃): δ 1.32 (d 6H, −CH₃), 1.22 (d 6H, −CH₃), 2.08 (hept 2H,

−CH), 7.51 (mult 2H, aromatic CH), 7.32 (mult 3H, aromatic CH).

¹³C NMR (CDCl₃): δ = 20.1, 21.9, 22.1, 128.3, 128.5, 136.5.

Cu₄(I)₄(Sb^tBu₃)₄(3). Copper iodide (0.08 g, 0.42 mmol) was added

to Sb^tBu₃(0.5 g, 1.7 mmol) in 15 mL of ﬂuorobenzene at −20 °C

under argon atmosphere. The reaction was allowed to warm to room

temperature, and the reaction mixture was stirred overnight to provide

a colorless solution and a white precipitate. The solution was

decanted, and the white solid was washed several times with pentane

to remove impurities, leaving behind the puriﬁed desired product as a

white solid (0.051 g, 0.03 mmol). Yield: 25%. Crystals suitable for X-

ray crystallography were obtained via slow vapor diﬀusion of pentane

Dimethylphenylantimony (SbMe₂Ph). The methylmagnesiu-

miodide Grignard solution was generated in situ in the same manner

as above with magnesium turnings (4.08 g, 168 mmol) and

iodomethane (19.7 g, 139 mmol). Trichloroantimony (8.45 g, 37.1

mmol) and triphenylantimony (6.55 g, 18.5 mmol) were stirred

together (no solvent) for 3 h to aﬀord dichlorophenylantimony (15.0

g, 55.6 mmol) in quantitative yield. The SbCl₂Ph was dissolved in

Et₂O (125 mL) and added dropwise to the ﬁltered Grignard solution,

and the reaction was reﬂuxed for 3 h. The solution was then quenched

with degassed water (70 mL). Under inert atmosphere, the organic

layer was separated, dried over sodium sulfate, and ﬁltered. The

solvent was removed in vacuo to produce a slightly yellow oil. A

distillation was performed to aﬀord the desired product (50 °C, 0.01

1

into a DCM solution of the product at −20 °C. H NMR (CDCl₃):

1.47 (s 18H, −CH₃). ¹³C NMR (CDCl₃): δ = 32.6, 35.6. Anal. cald

(% wt.) for Cu₄I₄Sb₄C₄₈H₁₀₈: C, 29.81; H, 5.63. Found: C, 29.78; H,

5.74.

Cu₄(I)₄(Sb^tBu₂Ph)₄(4). Copper iodide (0.08 g, 0.42 mmol) was

added to Sb^tBu₂Ph (0.53 g, 1.7 mmol) in 15 mL of ﬂuorobenzene at

−20 °C under argon atmosphere. The reaction was allowed to warm

to room temperature, and the reaction mixture was stirred overnight

to yield a clear, colorless solution. The solution was ﬁltered, and the

solvent was removed in vacuo to generate a light yellow oil. The oil

was washed several times with pentane, which precipitated out the

product as a white solid (0.065 g, 0.03 mmol). Yield: 31%. Crystals

suitable for X-ray crystallography were obtained via slow vapor

diﬀusion of pentane into a DCM solution of the product at −20 °C.

¹H NMR (CDCl₃): 1.49 (s 18H, −CH₃), 7.33 (mult 3H, aromatic

CH), 7.90 (mult 2H, aromatic CH). ¹³C NMR (CDCl₃): δ = 32.0,

34.7, 128.5, 129.1, 134.0, 138.1. Anal. cald (% wt.) for

Cu₄I₄Sb₄C₅₆H₉₂: C, 33.39; H, 4.60. Found: C, 33.17; H, 4.62.

Cu₂(I)₂(SbⁱPr₂Ph)₄(5). Copper iodide (0.04 g, 0.22 mmol) was

added to SbⁱPr₂Ph (0.26 g, 0.88 mmol) in 15 mL of ﬂuorobenzene at

−20 °C under argon atmosphere. The reaction was allowed to warm

to room temperature, and the reaction mixture was stirred overnight,

yielding a clear, colorless solution. The solvent was removed in vacuo

to yield a light brown oil and a white precipitate. The oil was dissolved

into pentane and placed into the freezer at −20 °C to yield the dimer

product as a white crystalline solid (0.03 g, 0.02 mmol). Yield: 16%.

Crystals suitable for X-ray crystallography were obtained via slow

evaporation of a pentane solution of the product at −20 °C. ¹H

NMR: δ 1.39 (d 6H, −CH₃), 1.28 (d 6H, −CH₃), 2.24 (hept 2H,

−CH), 7.60 (mult 2H, aromatic CH), 7.32 (mult 3H, aromatic CH).

¹³C NMR (CDCl₃): δ = 20.1, 21.9, 128.5, 128.8, 136.3, 136.5. Anal.

cald (% wt.) for Cu₂I₂Sb₄C₄₈H₇₆: C, 37.90; H, 5.04. Found: C, 37.46;

H, 4.56.

1

mmHg) as a colorless oil (8.9 g, 39.0 mmol). Yield: 70%. H NMR

(CDCl₃): δ 0.97 (s 6H, −CH₃), 7.55 (mult 2H, aromatic CH), 7.32

(mult 3H, aromatic CH). ¹³C NMR (CDCl₃): δ = 128.3, 128.6, 135.0,

137.5.

Ditertbutylphenylantimony (Sb^tBu₂Ph). tert-Butyl magnesium

chloride solution (2.0 M in Et₂O, 20.8 mL, 41.7 mmol) was added

dropwise to a stirring solution of SbCl₂Ph in Et₂O (5 g, 18.5 mmol,

125 mL) on ice. The solution was reﬂuxed for 3 h and then allowed to

cool to room temperature. The solution was quenched with degassed

water (70 mL). Under inert atmosphere, the organic layer was

separated, dried over sodium sulfate, and ﬁltered. The solvent was

removed in vacuo to produce a slightly yellow oil. A distillation was

performed at 130 °C to aﬀord the product as a colorless clear oil (1.7

1

g, 5.4 mmol). Yield: 30%. H NMR (CDCl₃): 1.31 (s 18H, −CH₃),

7.31 (mult 3H, aromatic CH), 7.61 (mult 2H, aromatic CH). ¹³C

NMR (CDCl₃): δ = 31.6, 34.6, 128.4, 129.1, 135.6, 137.5.

Tri-tert-butylantimony (Sb^tBu₃). A solution of SbCl₃in Et₂O (3

g, 13.1 mmol, 100 mL) was added dropwise to a stirring solution of

tert-butyl magnesium chloride (2.0 M in Et₂O, 26.3 mL, 52.6 mmol)

on ice. The solution was reﬂuxed for 3 h and then allowed to cool to

room temperature. The solution was quenched with degassed water

(70 mL). Under inert atmosphere, the organic layer was separated,

dried over sodium sulfate, and ﬁltered. The solvent was removed in

vacuo to produce a slightly yellow oil. A distillation was performed at

100 °C to provide the product as a slightly yellow oil (1.0 g, 3.4

1

mmol). Yield: 26%. H NMR (CDCl₃): 1.33 (s 18H, −CH₃). ¹³C

NMR (CDCl₃): δ = 32.3, 34.7.

Cu₂(I)₂(SbMe₂Ph)₄(6). Copper iodide (0.14 g, 0.75 mmol) was

added to SbMe₂Ph (0.68 g, 3.00 mmol) in 15 mL of ﬂuorobenzene at

−20 °C under argon atmosphere. The reaction was allowed to warm

to room temperature, and the reaction mixture was stirred overnight

to generate a clear, colorless solution. The solvent was removed in

vacuo to aﬀord a clear oil. The product was dissolved into pentane

and placed into the freezer at −20 °C to yield the product as a white

solid (0.11 g, 0.08 mmol). Yield: 23%. Crystals suitable for X-ray

crystallography were obtained via slow evaporation of a pentane

Tricyclohexylantimony (SbCy₃). A solution of SbCl₃in Et₂O

(2.5 g, 10.9 mmol, 100 mL) was added dropwise to a stirring solution

of cyclohexyl magnesium chloride (1.0 M in Et₂O, 38.3 mL, 38.3

mmol) on ice. The solution was reﬂuxed for 3 h and then allowed to

cool to room temperature. The solution was quenched with degassed

water (70 mL). Under inert atmosphere, the organic layer was

separated, dried over sodium sulfate, and ﬁltered. The solvent was

removed in vacuo to produce a white solid (2.7 g, 7.2 mmol). Yield:

1

66%. H NMR (CDCl₃): 1.31 (mult 12H, −CH₂), 1.48 (mult 3H,

1

−CH), 1.73 (mult 18H, −CH₂). ¹³C NMR (CDCl₃): δ = 27.1, 28.2,

29.0, 32.9.

solution of the product at −20 °C. H NMR: (C₆D₆): 0.85 (s 6H,

−CH₃), 7.10 (mult 3H, aromatic CH), 7.53 (mult 2H, aromatic CH).

¹³C NMR (CDCl₃): δ = −1.7, 128.8, 128.9, 134.7, 135.1. Anal. cald

(% wt.) for Cu₂I₂Sb₄C₃₂H₄₄: C, 29.64; H, 3.42. Found: C, 29.41; H,

3.47.

Synthesis of the Metal Complexes. Cu(I)₄(SbⁱPr₃)₄(1). This

cluster was previously synthesized and reported in our previous

communication.²²

Cu₄(I)₄(SbCy₃)₄(2). Copper iodide (0.14 g, 0.74 mmol) was added

to SbCy₃(0.28 g, 0.74 mmol) in 15 mL of ﬂuorobenzene at −20 °C

under argon atmosphere. The reaction was allowed to warm to room

temperature, and the reaction mixture was stirred overnight. The

clear, slightly yellow solution was ﬁltered, and the solvent was

removed in vacuo to aﬀord a light yellow oil and a white precipitate.

The product was washed several times with pentane to remove the oil,

Cu₂(I)₂(SbPh₃)₄(7). The product Cu₂(I)₂(SbPh₃)₄was prepared

according to a published procedure.²⁸¹H NMR: (C₆D₆): 7.23 (mult

2H, aromatic CH), 7.34 (mult 3H, aromatic CH). ¹³C NMR

(CDCl₃): δ = 128.8, 129.1, 136.4, 138.5. Anal. cald (% wt.) for

Cu₂I₂Sb₄C₇₂H₆₀: C, 48.23; H, 3.37. Found: C, 45.10; H, 3.25.

Cu₄(I)₄(SbⁱPr₂Ph)₄(8). Copper iodide (0.04 g, 0.22 mmol) was

added to SbⁱPr₂Ph (0.06 g, 0.22 mmol) in 15 mL of ﬂuorobenzene at

C

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

−20 °C under an inert argon atmosphere. The reaction was allowed

to warm to room temperature, and the reaction mixture was stirred

overnight to yield a clear, colorless solution. The solvent was removed

in vacuo to yield a light brown oil and a white precipitate. The

product was washed several times with pentane to remove the oil,

leaving the desired product as a white solid (0.09 g, 0.04 mmol).

othersonly SbPh₃has been used speciﬁcally in copper

metalations; such studies only obtained monomeric and

dimeric structures of CuX (X = Cl, Br, I),^38,41−44rather than

the cuboid type structures of interest in this work. Thus,

overall, with the stated exception of SbPh₃, none of the copper-

binding parameters or structural motifs derived from

homoleptic and heteroleptic SbR₃ligands have been reported.

Electronic Ligand Eﬀects. We targeted ligands sterically

both larger and smaller than SbⁱPr₃, and with stronger and

weaker donor strengths to investigate the full range of ligand

eﬀects on emission wavelength. Speciﬁcally, a previous study

on electronic ligand eﬀects in phosphine-supported thermo-

chromic cuboids determined that while the ligand donor

strength had the expected eﬀect on the high-energy (HE,

³XLCT) emission (weaker ligand = lower energy emission),

there was no correlation with low-energy (LE, ³CC)

emission.⁴⁵As only the LE band was observed in the single

example of a published Sb₄Cu₄I₄cuboid (1 derived from

SbⁱPr₃), we postulated that the primary eﬀect of altering the

organic substituents on the antimony donor atom would be

steric rather than electronic. Additionally, the lack of access to

a family of Sb₄Cu₄I₄cuboids derived from para-substituted

SbPh₃derivatives (R = OMe, CH₃, H, CF₃)⁴⁵due to the

inability of SbPh₃to support the cuboid motifprevented the

development of a classical Hammett relationship between

donor strength and emission properties.

Second, the inclusion of a single phenyl unit in the ligand

design would greatly amplify the breadth of synthetically

accessible heteroleptic ligands with relative ease. In the pursuit

of exclusively NIR emitters, we carefully considered whether a

proposed series of SbR₂Ph ligands (R = alkyl) would impart

the undesirable HE band observed in PPh₃-based Cu₄I₄cubes.

For example, nearly all reported phenylphosphine-supported

copper-halide cubes exhibit LE → HE thermochromism (high

temp → low temp) due to the nb(I) → π*(Ph) XLMCT.^17,19

However, deeper inspection of the literature reveals that the

presence of two phenyl substituents (e.g., PPh₂Pr, PPh₂OEt,

PPh₂CH₂CHCH₂)^17,46,47is required to imbue thermo-

chromism, whereas one phenyl unit is insuﬃcient. For example,

the PPhMeⁱPr supported cube is simply luminescent (LE

emission only, RT → 77 K).⁴⁸Similarly, no detectable HE

emission was observed in a Cu₄I₄cluster without any aromatic

substituent (L = Pcyp₃; cyp = cyclopentyl), but the cluster did

exhibit thermoluminescence (like 1 derived from SbⁱPr₃).⁴⁹

Notably, there are several other examples of alkyl-only

phosphorus-based cubes, but these were not investigated for

their luminescent properties.^50,51Therefore, we conﬁdently

pursued the synthesis of several “monophenyl” substituted

SbPhR₂ligands with the premise of only steric consequences

on LE emission.

1

Yield: 51%. H NMR: δ 1.48 (d 6H, −CH₃), 1.35 (d 6H, −CH₃),

2.38 (hept 2H, −CH), 7.70 (mult 2H, aromatic CH), 7.33 (mult 3H,

aromatic CH). ¹³C NMR (CDCl₃): δ = 20.8, 21.9, 128.6, 128.9,

136.4, 136.8. Anal. cald (% wt.) for Cu₄I₄Sb₄C₄₈H₇₆: C, 30.31; H,

4.03. Found: C, 29.79; H, 3.98.

Physical Measurements. NMR (¹H and ¹³C) measurements

were obtained using a 500 MHz Bruker AVANCE III NMR; CDCl₃

was referenced to 7.26 and 77.2 ppm for ¹H and ¹³C spectra,

respectively. Elemental analyses (C, H) were performed by Midwest

Microlab, IN. Absorption spectra were recorded on a Varian Cary

6000i UV−vis−NIR spectrophotometer using Starna Quartz Fluor-

ometer Cells with path lengths of 10 mm. Luminescence measure-

ments were recorded on a Photon Technology International QM 4

spectroﬂuorimeter with a BryteBox interface using FeliX32 software.

Luminescence spectra were excited using a xenon short-arc lamp

(USHIO, UXL-75XE) and recorded with a PTI detection system

(model 814) and a photomultiplier tube (Hamamatsu, R928P)

connected to a PTI lamp power supply (LPS-250B). Time-resolved

spectra were excited with a xenon ﬂash lamp (Hamamatsu, L4633)

and recorded using a photomultiplier tube (Hamamatsu, R562)

connected to a PTI XenoFlash power supply.

X-ray Data Collection. For complexes 2, 3, 4, and 7, the X-ray

diﬀraction data were collected on a Rigaku AFC12 diﬀractometer with

a Saturn 724+ CCD using a Bruker AXS Apex II detector and a

graphite monochromator with Mo Kα radiation (λ = 0.71073 Å). Low

temperatures were maintained using an Oxford Cryostream low

temperature device. Data reduction was performed using the Rigaku

Crystal Clear version 1.40.²⁹Structures were solved by direct methods

using SHELXT³⁰and reﬁned by full-matrix least-squares on F²with

anisotropic displacement parameters for the non-H atoms using

SHELXL-2014/7.³¹Structure analysis was aided by use of the

programs PLATON³²and WinGX.³³For complexes 5 and 6, the X-

ray diﬀraction data were collected at −173 °C on a Nonius Kappa

CCD diﬀractometer using a Bruker AXS Apex II detector and a

graphite monochromator with Mo Kα radiation (λ = 0.71073 Å). Low

temperatures were maintained using an Oxford Cryosystems 700 low-

temperature device. Data reduction was performed using SAINT

V8.27B.³⁴The structure was solved by direct methods using

SHELXT³⁰and reﬁned by full-matrix least-squares on F²with

anisotropic displacement parameters for the non-H atoms using

SHELXL-2014/7.³¹Structure analysis was aided by use of the

programs PLATON³²and WinGX.³³

DFT, Tolman Angle, and Percent Buried Volume (%V_B)

Calculations. The program Fireﬂy³⁵was used for DFT calculations

using the B3LYP functional with the following basis sets per atom: Sb,

TZP; I, 6-311G; Cu, C, and H, 6-31G**. The ground state structure

was geometry optimized from the X-ray coordinates, and the excited

state triplet was, in turn, optimized from the ground state structure.

The resulting structures were visualized with MacMolPlt.³⁶The

orbital energy diagram was constructed in Mathematica. Tolman cone

angles were calculated using the Mathematica package FindConeAn-

gle developed by Allen et al.³⁷Percent buried volumes were calculated

using SambVca (Cavallo et. al).⁴¹

Steric Ligand Eﬀects. We hypothesized that as the steric

bulk of the ligand decreased, the {Cu−SbR₃} unit would be

drawn closer to the cluster center due to fewer steric

interactions with the neighboring SbR₃ligands and the iodide

ions. Consequently, we postulated smaller size would decrease

the average Cu−Cu bond distance, thereby red-shifting the

RESULTS AND DISCUSSION

■

Synthesis and Rationale. Synthetic Novelty. We

previously reported that several of the ligands used herein

(SbMe₂Ph, SbⁱPr₂Ph, and SbⁱPr₃) bind and structurally stabilize

nickel(II).³⁸In addition, several ligands employed in this work

were previously synthesized by others (SbCy₃, Sb^tBu₃,

Sb^tBu₂Ph)^39,40or can be purchased commercially (SbPh₃).

While several of these ligands have been used in metalations

with other 3d transition metalsboth in our work and

3

emission derived from the CC excited state in accordance

with published trends.^19,22,45,52Although our work is

exclusively performed with antimony ligands, these ﬁndings

might apply to any copper-based luminescent clusters.

Synthetic Method of Ligands and Clusters. Antimony

ligands with only one aryl group, such as SbMe₂Ph and

SbEt₂Ph, have been synthesized previously, and others can be

D

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

synthesized by following a similar procedure.^53,54Each ligand

was synthesized by reaction of the appropriate Grignard

reagent with either SbCl₃or SbCl₂Ph (crystallized to improve

purity). The heteroleptic ligands (SbMe₂Ph, SbⁱPr₂Ph,

Sb^tBu₂Ph) and Sb^tBu₃required distillations for puriﬁcation,

while the homoleptic ligands did not require additional

puriﬁcation. The copper metalation conditions in ﬂuoroben-

zene (FPh) are similar to that previously reported for 1

(SbⁱPr₃-based). However, the tert-butyl based clusters (3 and

4) precipitated from FPh, while cluster 2 proved insoluble in

pentane during washing of the crude solid. A summary of the

synthetic preparations of the ligands and metal complexes is

depicted in Schemes 1 and 2, respectively.

coordinate in the case of 7, while only one SbⁱPr₃ligand is

bound per copper atom in cluster 1, implying that SbⁱPr₃is

sterically larger. This discrepancy can be rationalized by

examining a diﬀerent measure of ligand sterics, namely the

percent buried volume (%V_B), a parameter that is regarded to

more accurately deﬁne the steric bulk of a ligand.⁵⁵The

percent buried volumes of the four ligands were calculated

using SambVca (Cavallo et. al),⁵⁶aﬀording the following

values: SbMe₂Ph = 23.3% < SbPh₃= 25.8% < SbⁱPr₂Ph =

26.0% < SbⁱPr₃= 27.0% (Table 1). Thus, the %V_Bvalues more

accurately reﬂect the functional nature of the ligand sterics and

compare favorably to the steric rankings of the analogous

phosphine ligands.⁵⁷

The %V_Bwas equally accurate in predicting the propensity

for larger ligands (cone angle ∼ ≥155°; %V_B> 27.0%) to form

the desired cuboid structures (see X-ray section, next

paragraph); there were no exceptions. The traditional cone

angle method provided the following values: SbCy₃, 152° <

SbⁱPr₃, 155° < Sb^tBu₂Ph, 161° < Sb^tBu₃, 163°. [Note: our

calculated ligand cone angles diﬀer slightly from previously

reported numbers (SbCy₃, Sb^tBu₂Ph = 166°; Sb^tBu₃= 177°,

SbⁱPr₃= 157°)].⁵⁸However, the %V_Bcalculations for these

ligands provided a close match to the previously published

cone angles. Overall, the clear %V_Bcutoﬀ between cube and

dimer formation suggests that %V_Bis the most reliable

parameter.

Scheme 1. Synthetic Preparations of the Homoleptic and

Heteroleptic Antimony Ligands

Scheme 2. Synthetic Preparations of the Copper−Antimony

Cuboids and Dimers

Cuboid Structures. Cu₄(I)₄(SbCy₃)₄(2), Cu₄(I)₄(Sb^tBu₃)₄(3),

t

and Cu₄(I)₄(Sb Bu₂Ph)₄(4). The formation of dimers rather

than cuboids from smaller antimony ligands prompted us to

employ sterically bulkier antimony ligands. The crystal

structures of the resulting antimony-supported copper-iodo

cuboids 1−4 are depicted in Figure 2. At the core of each

cuboid is a copper-based tetrahedron wherein the four copper

atoms reside at the vertices. Each copper ion is bound to three

iodine atoms, which in turn are each ligated in η₃fashion to

three copper ions. Each copper vertex is externally capped by

an antimony ligand. The large steric bulk of the antimony

ligand restricts the ligation of one (weak) antimony ligand per

copper ion, which presumably drives the copper centers to

assemble the cluster core. [Note: herein crystallographically

unique bonds are presented in the conventional fashion; bond

distance averages are stated in the format avg s.d. for the n

number of bond instances in the cluster.] Cluster 2 ligated by

SbCy₃has an average Cu−Cu distance of 2.83 0.14. Å, an

X-ray Structures and Ligand Steric Eﬀects. Dimeric

Complexes. Cu₂(I)₂(SbⁱPr₂Ph)₄(5), Cu₂(I)₂(SbMe₂Ph)₄(6), and

Cu₂(I)₂(SbPh₃)₄(7). Following on our initial hypothesis that the

steric size of the ligand dictates emission wavelength, we

attempted to synthesize cuboids using smaller antimony

ligands. Despite analogous reaction conditions to the synthesis

of 1, this subset of antimony ligands bearing phenyl

substituents provided dimers (Figure 1) rather than cuboid

structures. The cone angles of the three ligands in this series

(SbMe₂Ph = 119° < SbⁱPr₂Ph = 147° < SbPh₃= 155°; SbⁱPr₃=

155°; see next paragraph for discussion of the two 155°

ligands) were calculated using the Cartesian coordinates from

the crystal structures with the Mathematica program Exact

Ligand Cone Angle.³⁷Such small cone angles allow two

ligands per copper atom, providing a dimer instead of a cuboid

(cuboid = 1 SbR₃ligand per copper). Thus, the evidence

suggests that the cuboid motif can only be accessed with

ligands that have a relatively large cone angle (∼ ≥155°).

Tolman Cone Angle vs Percent Buried Volume. An

apparent discrepancy occurs when examining the calculated

ligand cone angles for both SbPh₃and SbⁱPr₃. Both cone angles

were calculated to be 155°, yet two SbPh₃ligands were able to

average Cu−Sb distance of 2.51

0.008 Å, and an average

Cu−I bond distance of 2.68 0.02 Å. Despite being derived

from a homoleptic ligand similar to 1, the tetrahedral core of 2

is not completely symmetric as found in the 4-fold symmetric

Cu₄core of Cu₄(I)₄(SbⁱPr₃)₄(1, in cubic I23 space group), in

which the unique crystallographic Cu−Cu bond length is

2.761(3) Å. In fact, each Cu−Cu distance in 2 is distinct from

the other ﬁve Cu−Cu distances, with the shortest distance of

2.675(1) Å and the longest of 3.104(1) Å. In fact, the set of

Cu−Cu distances for 2 represents (i) the broadest range of

distances and (ii) both the shortest and the longest distances

found in any cluster herein. Thus, these two extremes of high

ligand symmetry and low crystallographic symmetry demon-

strate that the selection of homoleptic ligand does not ensure

access to a highly symmetric Cu₄core in the solid state.

In fact, varying extents of Cu₄core symmetry can be

arbitrarily accessed with both homoleptic and heteroleptic

antimony ligands. For example, the homoleptic ligand Sb^tBu₃

gives rise to the partially asymmetric Cu₄core in

E

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 1. ORTEP diagrams (30% ellipsoids) for Cu₂(I)₂(SbⁱPr₂Ph)₄(5) (top left), Cu₂(I)₂(SbMe₂Ph)₄(6) (top right), and Cu₂(I)₂(SbPh₃)₄(7)

(bottom).

a

Table 1. Average Cu−Cu, Cu−Sb, and Cu−I Bond Distances (Å) for Complexes 1−7

Cu−Cu_avg

Cu−Cu_shortest

Cu−Cu_longest

Cu−Sb_avg

Cu−I_avg

Cone Angle (θ)

%V_B

Cuboids

Cu₄(I)₄(SbⁱPr₃)₄(1)

Cu₄(I)₄(SbCy₃)₄(2)

Cu₄(I)₄(Sb^tBu₃)₄(3)

Cu₄(I)₄(Sb^tBu₂Ph)₄(4)

Dimers

2.761(3)

2.675(1)

2.9478(6)

2.8037(7)

2.761(3)

3.104(1)

3.0032(6)

2.8618(8)

2.571(2)

2.707(2)

155

152

163

161

27.0

28.5

31.6

29.9

2.83 0.14

2.98 0.03

2.82 0.03

2.51 0.008

2.55 0.00004

2.5381(5)

2.68 0.02

2.69 0.006

2.69 0.01

Cu₂(I)₂(SbⁱPr₂Ph)₄(5)

Cu₂(I)₂(SbMe₂Ph)₄(6)

Cu₂(I)₂(SbPh₃)₄(7)

2.88 0.03

2.95 0.04

2.7255(5)

2.54 0.004

2.53 0.01

2.54 0.007

2.64 0.01

2.64 0.02

2.63 0.01

147

119

155

26.0

23.3

25.8

a

Also listed are the shortest and longest Cu−Cu bond distances for each complex, as well as the cone angle and percent buried volume parameter

for each ligand.

Cu₄(I)₄(Sb^tBu₃)₄(3), which exhibits two sets (n = 3) of

crystallographic Cu−Cu bonds at 2.9478(6) and 3.0032(6) Å.

Relatedly, the cuboid derived from heteroleptic Sb^tBu₂Ph

[Cu₄(I)₄(Sb^tBu₂Ph)₄, 4] also exhibits two sets of crystallo-

graphic Cu−Cu bonds at 2.8037(7) (n = 4) and 2.8618(8) Å

(n = 2). It is notable that the diﬀerences in bond length

between the shortest and longest Cu−Cu bonds in 3 and 4 are

not nearly as exaggerated as in 2. All of these data reinforce the

notion that ligand symmetry is not a (predictable) determinant

of crystallographic cluster symmetry. All relevant bond

distances for each of the three cuboids as well as

Cu₄(I)₄(SbⁱPr₃)₄are tabulated in Table 1.

The bond metric data indicates that irrespective of ligand

choice, ligand symmetry, or crystallographic symmetry, there

are only relatively small changes in the average Cu−Sb bond

distances (2.51−2.57 Å) and virtually no change in Cu−I

distances (2.68−2.71 Å). The Cu−Sb distances are typical for

copper complexes with antimony ligands (∼2.5 Å) and are

understandably longer than the average distances of Cu−P

(∼2.25 Å) or Cu−As (∼2.35 Å) bonds in the respective

analogues.^59,60Lastly, the Cu−I bond distances are akin to any

copper−pnictogen cuboid (∼2.6−2.7 Å); there is no clear

relationship between Cu−I distance and ligand design.^45,50

Luminescence. Each cuboid with Cu−Cu contacts was

tested for its luminescent properties. The excitation and

F

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 2. ORTEP diagrams (30% ellipsoids) for Cu₄(I)₄(SbⁱPr₃)₄(1, previously reported, top left),²²Cu₄(I)₄(SbCy₃)₄(2, top right),

t

Cu₄(I)₄(Sb^tBu₃)₄(3, bottom left), and Cu₄(I)₄(Sb Bu₂Ph)₄(4, bottom right).

emission proﬁles of four cuboids (1−4) at 77 K are depicted in

Figure 3 (1: SbⁱPr₃; 2: SbCy₃; 3: Sb^tBu₃; 4: Sb^tBu₂Ph).

Compared with phosphine-supported copper cubes at 77 K,

only a single emission is observed for 1−4analogous to the

low-energy (LE) band typically exhibited in the 500−600 nm

region for P₄Cu₄I₄cubes. Notably absent from the emission

spectra of 1−4 is the characteristic high energy (HE) band

found in thermochromic phosphine-supported cubes, which is

generally observed in the 400−500 nm range. At the ﬁrst

approximation, the lack of the HE band in 1−4 can be

attributed to the lack of phenyl groups as antimony

substituents (i.e, the phenyl-based nb(I) → (π*)Ph XLCT

triplet state is obviated). The single exception to this is cluster

4 (derived from Sb^tBu₂Ph); however, previous work using a

single-phenyl phosphine (namely, PPhⁱPrMe) also did not

display the HE band, suggesting that multiple aryl units are

required to activate the XLCT pathway, as explained and

hypothesized in the Electronic Ligand Eﬀects subsection.

Overall, considering the excitation proﬁles of the colorless

samples of 1−4 (λ_ex≈ 350; similar to phosphine cubes), this

results in a very large Stokes shift of ∼250 nm. Modulation of

the λ_exdid not result in any noticeable change in the emission

proﬁles of 1−4; namely, the appearance of the HE band was

never observed.

However, the room temperature emission spectra of 1−4 do

not exhibit any appreciable luminescence compared with the

low temperature spectra (Figure S11). This is in stark contrast

to the ambient temperature emission from phosphine-

supported cubes, in which a dominant LE band (500−600

nm) is preserved. An illustrative comparison is the case of

[Cu₄(I)₄(PPhⁱPrMe)₄], a phosphine-supported cube reported

by Harvey (with no XLCT) that exhibits room temperature

luminescence at 580 nm (LE band);⁴⁸the phenyl-free cluster

[Cu₄(I)₄(Pcpent₃)₄] reported by Perruchas exhibits similar

properties (λ_em≈ 520 nm).⁴⁹Such emission can be attributed

exclusively to the cluster-centered (³CC) emission mechanism.

Figure 3. Excitation and emission maxima for complexes 1−4 at 77 K.

G

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 4. Variable temperature (153−222 K) luminescence of clusters 2 and 4.

By comparison, our cluster 2 exhibits a similar (intermediate)

Cluster 3 was also tested for its luminescent properties,

emission energy (λ_em≈ 560 nm) and yet does not emit at

room temperature. Thus, the room temperature quenching of

emission in 1−4 cannot exclusively be due to the red-shifted

emission proﬁles. Although it can be postulated that lower

energy emissive states generally have shorter lifetimes due to

increased probability of nonradiative decay pathways,^61,62it is

clear in this case that the precise identity (P vs Sb) and

coordinates (long vs short bonds, vide infra) of atoms in this

family of clusters is critical in determining the accessible

photoactive states. On a similar note, these clusters are

nonemissive in the solution state at room temperature, likely

due to the aforementioned nonradiative decay pathways.

Further supporting the “suppression of nonradiative decay”

hypothesis, at low temperatures both pentane and 2-MeTHF

solutions of the clusters remain emissive.

wherein it exhibited a similar extent of luminescence to 2 and 4

at 77 K. However, variable temperature measurements under

the same conditions as those used for 2 and 4 at −120 °C (liq

N₂/pentane bath; 153 K) revealed a greatly diminished

intensity compared with the 77 K measurement. Thus, another

temperature bath regime was used at −152 °C (liq N₂/

isopentane; 121 K) to obtain the thermal decay of emission

intensity (Figure 5). The emission feature in the resulting data

To clearly showcase the precise temperature dependence

and the lack of multiple emissions at other temperatures,

variable temperature luminescence measurements were

collected. The variable temperature ﬂuorimetry data for

clusters 2 and 4 are depicted in Figure 4. The luminescent

LE bands for 2 and 4 are quite prominent at −120 °C (liq N₂/

pentane bath; 153 K), but for each cluster the luminescence

intensity decreases to baseline as the temperature increases to

298 K. There is a slight shift in the λ_emof 2 from 661 nm (low

temp) to 681 nm (RT), whereas a negligible λ_emshift occurs

for cluster 4 in the same temperature regime. The emission

from both 2 and 4 becomes essentially nonemissive above 230

K. The small shift in λ_emfor 2 (20 nm) is interesting but not

indicative of the authentic dual emission found in thermo-

chromic systems. Such small shifts in λ_em(< ∼50 nm) have

been observed in many cuboids and have been postulated to

originate from symmetry distortions inside the Cu₄core.⁶³⁻⁶⁵

Intriguingly, the structural data for 4 reveal some extent of

homogeneity among its Cu−Cu bond distances (4 × 2.80 Å; 2

× 2.86 Å), while cluster 2 exhibits six unique bond lengths

one for every Cu−Cu interaction. Similarly, the originally

reported cluster 1which exhibits complete homogeneity of

Cu−Cu contacts: 6 × 2.76 Åalso did not exhibit any

signiﬁcant shift in λ_emacross variable temperatures.²²Thus, our

data is consistent with the previously asserted connection

between cluster symmetry and a small, temperature dependent

wavelength shift in the LE emission.⁶³⁻⁶⁵

Figure 5. Variable temperature (123−233 K) luminescence of cluster

3.

for 3 rapidly declines as a function of increasing temperature,

and the emission intensity when warmed from −152 to −120

°C is approximately 25% of the emission intensities from 2 and

4 at the same temperature.

Structure−Luminescence Correlations. Temperature

Dependent Structures and Emission. The marked diﬀerence

of 3 in its diminished luminescence intensity (from 1, 2, and 4)

motivates the exploration of any possible correlation between

structural parameters and luminescence properties. Out of all

of the clusters studied herein, cluster 3 is most notable in that

it exhibits the longest average Cu−Cu distances (2.98 Å) by a

wide marginsigniﬁcantly longer than the next closest Cu−

H

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 6. For cuboids 1 (left) and 3 (right), correlation between the temperature dependent changes in X-ray structure (Cu−Cu, Cu−Sb, or Cu−I

bond distances) and percent of maximum luminescence intensity (red data points).

Table 2. Table of Ligand Cone Angles, Average Cu−Cu Bond Distances, Emission Wavelengths, and Other Pertinent Bond

Distances for Complexes 1−4

Cu₄(I)₄(SbⁱPr₃)₄(1)

Cu₄(I)₄(SbCy₃)₄(2)

Cu₄(I)₄(Sb^tBu₂Ph)₄(4)

Cu₄(I)₄(Sb^tBu₃)₄(3)

%V_B

Emission (λ)

27.0

711

28.5

666

29.9

575

31.6

558

Cu−Cu avg (Å)

Shortest Cu−Cu

Longest Cu−Cu

# of unique (crystallographic) Cu−Cu bonds

2.761

1

2.826

2.679

3.097

6

2.822

2.803

2.861

2

2.975

2.997

3.003

2

Cu_avgvalue (2.83 Å, for 2). Previously, it was suggested that 2

× the van der Waals radius of copper (2.80 Å) was the

theoretical cutoﬀ for copper-centered LE emission, as beyond

that distance cuprophilic interactions cease.^65,66However, that

has been disproven in the case of phosphine-supported cubes,

wherein LE emission (λ_em= 500 nm) has been observed up to

Cu−Cu_avg= 3.47 Å, as in the case of [Cu₄(I)₄(tmp)₄]where

tmp = tris(3-methylphenyl)phosphine.⁶⁷

However, all of the Cu−Cu distances found in 3 are longer

than those for 1 irrespective of any temperature considerations.

Regarding the emission, it is important to note that both 1 and

3 exhibit comparably strong luminescence at 77 K, but they

exhibit diﬀerent characteristic “turn-on” temperatures (T_em)

upon cooling. The T_emwas collected by determining the ratio

of the luminescence intensity at varying temperatures

(collected during the variable temperature luminescence

measurements) to the intensity at 77 K, aﬀording a parameter

deﬁned as “percent max intensity”. A full graphical

representation of each cuboid’s percent max intensity at

varying temperatures along with a more detailed explanation of

the process to determine this parameter can be found in Figure

S12. Despite the very low temperature used in a controlled

cooling bath (isopentane/liq N₂; 123 K), cluster 3 remains

weakly emissive compared with 1 at an even higher

temperature (pentane/liq N₂; 163 K). As a result, the exact

emission “turn-on” temperature remains elusive for 3 but is

likely in the 100−125 K range, which is correlated with Cu−

Cu_avg≈ 2.96 Å. It is thus evident that the 2.80 Å van der Waals

“cutoﬀ” is not a litmus test indicator for luminescence, but

instead there is a correlation between the Cu−Cu_avgand

characteristic T_emvalue (shorter Cu−Cu_avg= higher T_em).

Eﬀect of Phenyl Substituents on Thermochromism versus

Thermoluminescence. Complexes 1, 2, and 3 lack any

aromatic character in the ligand backbone, which has been

In the previous communication, crystal structures of 1

(derived from SbⁱPr₃) solved across a range of temperatures

(100−300 K) established a correlation between Cu−Cu bond

length and emission intensity; for clarity, an updated version of

this plot is presented herein as Figure 6 (left). It was notable

for 1 that the only signiﬁcant changes in bond distances from

100 to 300 K occurred in the Cu−Cu contacts, and the Cu−I

and Cu−Sb bond lengths remained relatively invariant. Due to

the vastly diﬀerent temperature proﬁle of 3 (temperature of

emission “turn-on”, T_em= 100−125 K) compared with the

other cuboids (1, 2, and 4: T_em≈ 200 K), we determined the

structure−luminescence correlation for 3 using the same

variable temperature X-ray structure method (Figure 6, right).

Regarding the temperature dependent X-ray structure

metrics, the same general trends are observed for 3 as for 1.

The Cu−I and Cu−Sb bonds remain relatively constant across

the 200 K range. As cluster 3 possesses two sets of

crystallographic Cu−Cu distances, each individual data set is

plotted (gray) as well as the average distances and trendline

(black). Notably, the change in Cu−Cu distances for 3 across

the temperature range is 0.08 Å, nearly identical to that for 1.

3

proven necessary to observe the competing CC↔³XLCT-

(aryl) emissions that are present in dual-emitting thermochro-

mic compounds (a more detailed explanation can be found in

I

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 7. Graphical summary plot of emission wavelength versus percent buried volume.

previous sections). Therefore, purely thermoluminescent

properties are expected and, indeed, observed. Regarding

cluster 4 (L = Sb^tBu₂Ph), the temperature dependent emission

(Figure 4) clearly exhibits a single emission feature at all

temperatures, consistent with the expectation of thermolumi-

nescence only. Thus, from the combination of low temperature

and variable temperature luminescence studies, it appears that

the “two-phenyl” rule for thermochromism remains constant

from phosphine to stibine systems. However, we do note that

the possible counter-example of an SbPh₂R-supported cuboid

has remained synthetically inaccessible at present.

Correlation between Steric Bulk and Emission Energy. A

correlation is evident between the steric size of the ligand and

the maximum emission wavelength (Table 2, Figure 7). Of all

cuboids 1−4, the lowest-energy emitting cuboid is the original

complex, Cu₄(I)₄(SbⁱPr₃)₄(1), with λ_em= 711 nm, which also

exhibits the lowest %V_Bat 27.0%. As tabulated in Table 2 and

illustrated in Figure 7, for clusters 1−4 an increasing %V_Bis

correlated with decreasing λ_emvalues vis a vis blue-shifted

emission energies. The cuboids synthesized with SbCy₃(%V_bur

= 28.5%), Sb^tBu₂Ph (%V_bur= 29.9%), and Sb^tBu₃(%V_B=

31.6%) have emission wavelengths of 661, 575, and 558 nm,

respectively. This trend is also structurally correlated to the

average Cu−Cu bond distances in the cluster, wherein

sterically smaller ligands result in shorter Cu−Cu_avgdistances

(lower emission energies). The correlation between smaller

ligands and shorter Cu−Cu_avgdistances likely arises because

the entire {Cu−SbR₃} unit is best able to approach the center

of the cluster when {Cu−SbR₃} is small. Notably, the overall

importance of steric eﬀects in this series of copper dimers/

clusters is evident in that any ligands smaller than SbⁱPr₂Ph

resulted in exclusive dimer formation (which accommodates cis

SbR₃ligands) rather than forming tetramers (SbR₃is too big

for the cis motif).

respectively) and yet exhibit a 100 nm diﬀerence in emission

wavelength (666 and 575 nm, respectively). This implies the

existence of another subtle structural reason for the vast

diﬀerence in emission energy. As Perruchas has demonstrated

that ligand donor strengths in phosphine-based cubes have no

trend-wise eﬀect on LE emission,⁴⁵we postulate that the

explanation can again be found in the structural metrics. Closer

inspection reveals that 2 exhibits the shortest Cu−Cu bond

(2.679 Å) found in any of the clusters. In contrast, the shortest

Cu−Cu bond in 4 resides at 2.803 Å. DFT calculations reveal

3

that the LUMO in the CC transition is due to an MO that is

highly delocalized across the cluster center, as opposed to an

individual lowest lying molecular orbital.^68,69While it has been

shown that short Cu−Cu bond distances do red-shift the

emission, it is not clear whether the presence of a few

particularly short Cu−Cu bonds in a cluster center will

similarly adjust the maximum emission wavelength in these

complexes.^70,71However, there is some precedent for this

phenomenon.^{64,65,71−73}Kim et. al synthesized two crystallo-

graphic isoforms of the same cubane (derived from 2-

(cyclohexylthio)-1-thiomorpholinoethanone) that diﬀered in

their solvation state (desolvated versus MeCN solvate). The

two structures had similar Cu−Cu_avgdistances (2.70 vs 2.71

Å), but the range of Cu−Cu bond lengths within the cluster

ranged from 2.607−2.773 Å for the desolvated cluster versus

2.643−2.771 Å for the CH₃CN solvated cluster. The presence

of the shorter Cu−Cu bond distance in the desolvated

structure resulted in a red-shifted λ_em= 600 nm, compared

with the 540 nm emission of the cluster with the longer shortest

Cu−Cu distance.⁷¹Similar results were reported in two other

cases, where although the two comparative compounds

displayed Cu−Cu_avgbond distances separated by less than

0.02 Å, one compound contained a signiﬁcantly shorter Cu−

Cu_minbond distance. In each case, the compound with the

shorter Cu-Cu_mindistance had a red-shifted λ_emby 40−50

nm.^73,74Overall, these examples plus the work described herein

(notably, 2 and 4) suggest that the average Cu−Cu distance

An interesting caveat to this data resides in the comparison

of cluster 2 with cluster 4: these clusters exhibit nearly identical

Cu−Cu_avgdistances (2.826

0.14 and 2.822

0.03 Å,

J

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 8. (Left) Excitation and emission features (77 K) for cluster 8 showing λ_em= 670 nm; (right) Variable temperature luminescence (143 to

253 K) for 8.

provides a first approximation rank-order correlation between

structure and emission wavelength (shorter distance = red-shifted

λ_em). However, when similar Cu−Cu_avgvalues are present, the

shortest Cu−Cu_minresults in the more red-shifted emission profile.

Correlation between Crystallographic Symmetry and

Emission Energy. Based on the metrics discussed above

showing that clusters supported by the smallest SbR₃ligand

resulted in the most red-shifted NIR emission, we synthesized

the next smallest ligand in the seriesnamely SbⁱPr₂Ph.

Indeed, this intermediate-sized SbR₃ligand was the only case

where we isolated both a dimer (5) and tetranuclear cluster

(8) by varying the ligand equivalents (excess versus 1:1,

respectively). This suggests that the %V_Bof SbⁱPr₂Ph (26.0%)

is of truly intermediate steric bulk between the small set [%V_B

≤ 25.8% (SbPh₃)] and large set [%V_B≥ 27.0% (SbⁱPr₃)] of

antimony ligands. Another (unintended) diﬀerence was the

lower symmetry of SbⁱPr₂Ph versus SbⁱPr₃. Unfortunately, due

to competing space groups of crystallization, the exact

structure of the SbⁱPr₂Ph cuboid 8 could not be precisely

determined, although several partial structure solutions

unambiguously support an asymmetric cuboidal structure

and Cu−Cu bond lengths in the 2.7−2.8 Å range and on

average are about equal to the 2.76 Å Cu−Cu bond distance in

1. A collection of ball-and-stick representations of the Sb₄Cu₄I₄

core of 8 are provided in Figure S13. Presumably as a result of

the lower symmetry of SbⁱPr₂Ph versus SbⁱPr₃, the partial

structures can be solved in triclinic, tetragonal, or rhombohe-

dral space groupsbut all of these are of lower symmetry than

that of SbⁱPr₃supported 1 (cubic, I23). As a corollary, while 1

exhibits just a single crystallographically unique Cu−Cu bond,

the partial structures of 8 reveal the following sets of unique

Cu−Cu bonds: six sets of 1 (triclinic), two sets of 3

(rhombohedral), or one set of 2 plus one set of 4 (tetragonal).

The luminescence properties of this cuboid (Figure 8) reveal

several intriguing insights. First, while 8 emits in the red-shifted

region (670 nm) in this series (558−711 nm), cluster 1 still

emits 40 nm further into the NIR. Additionally, 8 is the only

cluster that is quantitatively emissive at room temperature, as

evidenced by the VT and T_ememission proﬁles seen in Figure

8 and Figure S12. Additionally, lower temperatures blue-

shifted the emission energy by ∼15 nm, which can be

attributed to the symmetry-imposed distortions in the Cu core,

as explained previously in the case of complex 2 (vide supra

the Luminescence section and Figure 4). The average nominal

Cu−Cu bond distance in 8 is roughly ∼2.7 Å, which would

imply the emission of this complex to be similar to 1 if the

emission was derived purely from the length of the Cu−Cu

bonds in the cluster center. Overall, 8 clearly follows the λ_em

trend established by the ligand sterics and Cu−Cu bond

lengths found in 2, 3, and 4. Indeed, the result of this analysis

ﬁrmly places 1 as the outlier in the series. Notably, 1 is the only

cubic structure in the series with just a single crystallo-

graphically unique Cu−Cu bond.

A relationship between the LE emission and crystal

symmetry was ﬁrst suggested by Hardt and Pierre, who

noticed the presence of a 4-fold symmetry element led to red-

shifting of emissions.⁶³Later, Holt and co-workers synthesized

several luminescent clusters with general structure

Cu₄(I)₄(CH₃CN)₂(L)₂(L = aniline derivative). One complex

with L = 2,6-dimethylaniline had no internal symmetry

element, a Cu−Cu_avgbond distance of 2.706 Å, and λ_em

=

568 nm. Another complex with L = p-anisidine had Cu−Cu_avg

= 2.716 Å but did contain an internal 2-fold symmetry element.

The latter cluster displayed an emission of 608 nm,

signiﬁcantly red-shifted as compared to the lower symmetry

compound.⁶⁵In a separate instance, two polymorphs of the

luminescent cluster, Cu₄(I)₄(PPh₂OEt)₄, were synthesized

one with tetragonal symmetry and the other monoclininc.

Although the Cu−Cu_avgbond distances for the two were

drastically diﬀerent (3.056 Å for tetragonal, 2.871 Å for

monoclinic), the two compounds displayed remarkably similar

emission proﬁles for the LE emission (λ_em= 570 and 580 nm

for the tetragonal and monoclinic structures, respectively).⁴⁶

These examples, along with our current ﬁndings, provide

further evidence for the correlation between symmetry and

λ_maxfor the LE emission.

Correlation between Electronic Ligand Eﬀects and

Emission Energy. Recently, Perruchas et al. determined there

was no discernible correlation between the electronic proper-

ties of the ligand and the energy of emission originating from

the cluster center (³CC).⁴⁵Our results corroborate those

ﬁndings. Using the classical Tolman electronic parameter

(TEP) to deﬁne the electronic strength of our antimony

ligands, we have shown previously that the addition of a single

K

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

phenyl group does lower the TEP versus an all-alkyl analogue,

while higher aliphatic organic substituents increased the TEP

(i.e., isopropyl versus methyl).³⁸Although it is diﬃcult to

quantify the TEP for the bulkier ligands (e.g., Sb^tBu3, SbCy₃),

we can extract useful information by examining analogous

the ground and excited state were optimized using the B3LYP

functional with the following basis sets per atom: Sb, TZP; I, 6-

311G; Cu, C, and H, 6-31G**. The calculations were

performed without any symmetry impositions.

Figure 9 depicts the ensemble of MOs resulting from the S₀

(left) and T₁optimized geometries; both the α and β spin

manifolds of T₁are represented (middle and right,

respectively). From the MO diagram of the ground state

(S₀), the HOMO consists primarily of the Cu 3d-orbitals with

a small contribution from the iodine 5p orbitals. At slightly

lower energy, the occupied MOs are a cluster of orbitals

resulting from various admixtures of the Cu and I orbitals, with

small contributions from the Sb 5p set. The LUMO of the

ground state (S₀) consists almost entirely of Cu−Cu bonding

character (4s) with an incredibly symmetric proﬁle, with a

small extent of I parentage (5p). This highly symmetric orbital

was labeled A₁to reﬂect that property. The lack of any

contribution from the antimony R groups provides insight for

the lack of the HE band (XLCT, ligand-based) in the emission

proﬁle. The next higher energy group of unoccupied MOs

resides on the antimony atoms, with some extension onto the

isopropyl carbons. These data suggest that a second (HE)

emission band may be possible provided the presence of more

aromatic character on ligand. However, from the previous

discussion, we have determined it would require multiple (≥2)

aryl substituents to decrease the energies of those orbitals to a

level competitive with the copper-based emission.

phosphine compounds (P^tBu_3TEP= 2056 cm⁻¹; PCy_3TEP

=

2056 cm⁻¹).⁷⁵Thus, an approximate order of Sb-ligand

strength would follow as Sb^tBu₃= SbCy₃> Sb^tBu₂Ph > SbⁱPr₃

> SbⁱPr₂Ph. If electronic eﬀects were crucial in determining the

³CC λ_em, it would be expected that the emission energies

follow this trend as well. However, the emission energies do

not, in fact, follow this trend, as the emission for complex 1 is

further red-shifted versus complex 8 and similarly for

complexes 2 and 3. Although from both our results and

those of Perruchas, there does appear to be a trend between

increasing electron withdrawing ligand eﬀects and decreasing

Cu−Cu bond lengths, there does not appear to be a deﬁnitive

correlation between said eﬀects and the emission energy

originating in the Cu−Cu core.

Lastly, lifetime measurements were collected for all of the

clusters and are tabulated in Table 3. Based on the relatively

Table 3. Lifetime Measurements for Clusters 1−4 and 8 at

Low Temperature and Room Temperature

77 K

77 K λ_ex

(nm)

λ_em

Cluster

(nm)

77 K τ (μs) 298 K τ (μs)

Cu₄(I)₄(SbⁱPr₃)₄(1)

Cu₄(I)₄(SbⁱPr₂Ph)₄

(8)

380

369

711

670

7.7 0.4

7.7 0.05

1.2 0.09

0.36 0.05

The T₁state of 1 was geometry optimized in the S = 1

conﬁguration. Obtaining this geometry optimized structure

required a slow progression at low basis set levels away from

the S₀geometry, followed by higher level calculations that

converged on the ﬁnal calculated structure for the T₁state.

The conversion from ground state singlet to excited state

triplet often enacts large geometric distortions, especially

among the Cu−Cu and Cu−I bond lengths.^68,69The relevant

bond distance data are tabulated in Table 4, along with the

experimental distances acquired from the crystal structure. The

most notable diﬀerence between the DFT calculated ground

state for 1 and the X-ray structure of 1 is the discrepancies in

Cu−Cu bond lengths [DFT: 2.424 0.004; X-ray: 2.761(3)].

This highlights the fact that the Cu₄core is especially elastic

with respect to temperature, considering the temperature of X-

ray data collection (100 K) versus the “temperature” of DFT

calculation (0 K). Upon conversion from S₀to T₁, the excited

state exhibits contracted Cu−Cu contacts (2.36 0.04 versus

Cu₄(I)₄(SbCy₃)₄(2)

355

341

666

575

9.8 0.05

5.4 0.8

0.43 0.04

0.42 0.06

Cu₄(I)₄(Sb^tBu₂Ph)₄

(4)

Cu₄(I)₄(Sb^tBu₃)₄(3)

339

558

5.9 0.03

0.39 0.05

long microsecond lifetime in each case in a narrow range (5.4−

9.8 μs), it is clear that all of the clusters undergo the same

phosphorescence-based emission process. At room temper-

ature, the lifetimes decrease by an order of magnitude. No

apparent trend is evident between the ligands, complexes, or

emission wavelengths and the lifetimes. However, the stark

diﬀerence between the room temperature and low temperature

lifetimes partially explains the lack of luminescence at room

temperature. Additionally, the nonradiative decay pathways are

mitigated as the temperature is lowered, thus accessing the

phosphorescent emissions with longer lifetimes.

2.424

0.004), and expanded Cu−I bonds (3.09

0.29

versus 2.839 0.007) that compensate for the contracted Cu₄

core. There is little change in the Cu−Sb bond lengths

between ground and excited states, which is common even

among Cu−P based cubes.¹⁹Overall, it is clear from both the

calculated and experimental data that the origin of

luminescence for all the Cu−Sb clusters presented herein is

centered inside the cluster.

Most Importantly, the Highest SOMO Consists of an

Incredibly Symmetric and Delocalized Set of Cu 3d Orbitals,

with Minor Contributions from the Iodine 5p Orbitals. The

highest SOMO of T₁exhibits decreased energy relative to the

S₀LUMO by nearly 2.5 eV (both denoted with the A₁

symmetry label, reﬂecting the similar origins from the Cu₄

core). This phenomenon of energy stabilization of the Cu−Cu

electron density responsible for the CC excited state between

ground and excited states has been observed in calculations on

the analogous pyridine supported cube, with a similar degree of

Density Functional Theory (DFT) Calculations for 1.

DFT calculations undertaken on the ground state singlet

(based on X-ray structure) and excited state triplet (geometry

optimized S = 1 state) describe the electronic states available in

the luminescent copper cuboids, and they have been

performed and reported.^68,69The calculations performed on

one of the ﬁrst thermochromic compounds,

[Cu₄(I)₄(pyridine)₄] cluster, elucidated the origins of the

lower energy XLCT excited state and the higher energy CC

excited state and determined that the pyridine π* orbitals were

the LUMO responsible for the XLCT transition. Although this

is consistent with the lack of a second emission band in our

ﬁrst thermoluminescent compound (1), a full DFT study on

the singlet and triplet states of that and related compounds is

critical to understand the origins of the copper−antimony

cuboid luminescence. Thus, DFT calculations were performed

on the singlet and triplet states of 1. The geometries for both

L

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 9. Molecular orbital diagram of 1 showing the ground and excited states, along with selected orbital parentage images.

triplet excited state, showcasing the consistency between the

two calculations for cluster 1. The energy diﬀerence between

the S₀HOMO and the highest energy SOMO of T₁represents

the energy of the phosphorescent emission from the LE band

that is experimentally observed. The corresponding calculated

emission wavelength was determined to be 1604 nm, which is

quite red-shifted from the experimental value (711 nm). Such a

deviation from experimental spectroscopic energies is not

uncommon in DFT calculations on multinuclear transition

metal complexes, in general,⁷⁶and for B3LYP in particular.⁵⁹

Table 4. Relevant Bond Distances for the Calculated Singlet

and Triplet States, and the Associated Experimental (X-ray)

Bond Distances for 1

Cu−Cu

Cu−I

Cu−Sb

S₀(DFT)

T₁(DFT)

Experimental (X-

ray)

2.424 0.004

2.36 0.04

2.761(3)

2.839 0.007

3.09 0.29

2.707(2)

2.544 0.002

2.52 0.02

2.571(2)

energy stabilization in the pyridine case (∼2.5 eV).⁶⁹For the

pyridine cube, the stabilization lowers the energies of these

orbitals past the π* orbitals of the pyridine ring, which are the

original LUMOs in the ground state and the LUMOs for the

second (HE) transition (X/MLCT). In contrast, our system

exhibits orbital properties similar to other cubes without

phenyl substituents, and the calculated MO diagram appears

similar, too.⁴⁹These systems display a unique single low-lying

triplet state, and it is clear that occupation of only one triplet

state is plausible. The LUMO of the T₁excited state in 1 (the

excited state of the excited state) displays signiﬁcant Sb

character, with an additional large amount of electron density

located along the Cu−I core. The LUMO+4 is comprised

primarily of Sb 4p orbitals with some additional interaction

with the Cu 3d orbitals, which interestingly is very similar to

the LUMO+1 cluster of orbitals in the ground state. The

previous HOMO of the ground state, consisting of Cu (3d)

and I (5p) parentage, is the new HOMO−2 cluster in the

CONCLUSION

■

In conclusion, the two critical orbitals involved in the S₁→ T₁

intersystem crossing processas well as the T₁→ S₀emission

processare highly symmetric, Cu(3d)-based MOs whose

delocalization and resultant energy are dependent on the

symmetry of the cluster. Such interpretation of the DFT

calculations is consistent with the experimental observation

that the cluster with the highest symmetry (1) allows for the

population of the most symmetric and lowest energy MOs in a

Cu₄I₄Sb₄system. We conclude that the extremely high

crystallographic symmetry of 1in conjunction with the

small %V_Bof SbⁱPr₃and corresponding short Cu−Cu_avgbond

distancesresults in cluster 1 exhibiting the lowest energy

NIR emission (711 nm) in this series of clusters. This work

convincingly relays that future designs for NIR emissive Cu₄

clusters must obtain shorter Cu−Cu bonds while retaining the

M

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

highest possible (cubic) crystallographic symmetrylikely via

the clever design of smaller, cuboid-supporting homoleptic

ligands derived from antimony or other main group elements.

Several additional and important trends and conclusions can

be explicitly listed as follows:

Advancement (Cottrell Scholar Award, RCSA 23640), the

American Chemical Society (PRF 53542-DN13), the National

Science Foundation (1 S10 OD021508-01), and UT Austin

College of Natural Sciences.

(1) Similar to other work on copper cubes, we ﬁnd that

cubes derived from ligands with 0 or 1 phenyl unit(s) do

not exhibit thermochromism; rather, only thermolumi-

nescence is observed.

REFERENCES

■

(1) Elie, M.; Renaud, J.-L.; Gaillard, S. N-Heterocyclic Carbene

Transition Metal Complexes in Light Emitting Devices. Polyhedron

2018, 140, 158−168.

t

(2) Antimony ligands with large R groups (ⁱPr₃, Bu₃,

(2) Zhang, K. Y.; Yu, Q.; Wei, H.; Liu, S.; Zhao, Q.; Huang, W.

Long-Lived Emissive Probes for Time-Resolved Photoluminescence

Bioimaging and Biosensing. Chem. Rev. 2018, 118, 1770−1839.

(3) Shan, X.-C.; Jiang, F.-L.; Chen, L.; Wu, M.-Y.; Pan, J.; Wan, X.-

Y.; Hong, M.-C. Using Cuprophilicity as a Multi-Responsive

Chromophore Switching Color in Response to Temperature,

Mechanical Force and Solvent Vapors. J. Mater. Chem. C 2013, 1,

4339−4349.

^tBu₂Ph, Cy₃) exclusively provide cuboid structures. In

contrast, smaller R groups (Ph₃, Me₂Ph) provide dimeric

species (not luminescent). Ligands with intermediately

sized R-groups (ⁱPr₂Ph) can access both structural

motifs.

(3) Regarding ligand sterics, percent buried volume (%V_B)

is superior to the conventional Tolman cone angle

metric in terms of accurately reﬂecting the functional

size of ligands that support cuboids (large) versus dimers

(small).

(4) The combination of short Cu−Cu bond and a Cu₄core

possessing highest crystallographic symmetries is

required for NIR emission.

(5) Among cubes with similar average Cu−Cu bonds, the

presence of a single short Cu−Cu bond red-shifts the λ_em

value.

(6) Lastly, shorter average Cu−Cu bonds (regardless of

symmetry) lead to higher T_emvaluesthat is,

luminescence occurs at temperatures closer to ambient

conditions.

(4) Reineck, P.; Gibson, B. C. Near-Infrared Fluorescent Nanoma-

terials for Bioimaging and Sensing. Adv. Opt. Mater. 2017, 5,

1600446−1600472.

(5) Wang, W.; Lei, X.; Gao, H.; Mao, Y. Near-Infrared Quantum

Cutting Platform in Transparent Oxyfluoride Glass−Ceramics for

Solar Sells. Opt. Mater. 2015, 47, 270−275.

(6) Liu, Y.; Zhang, P.; Fang, X.; Wu, G.; Chen, S.; Zhang, Z.; Chao,

H.; Tan, W.; Xu, L. Near-Infrared Emitting Iridium(III) Complexes

for Mitochondrial Imaging in Living Cells. Dalt. Trans. 2017, 46,

4777−4785.

̈

(7) Otto, S.; Dorn, M.; Forster, C.; Bauer, M.; Seitz, M.; Heinze, K.

Understanding and Exploiting Long-Lived Near-Infrared Emission of

a Molecular Ruby. Coord. Chem. Rev. 2018, 359, 102−111.

(8) Wagenknecht, P. S.; Ford, P. C. Metal Centered Ligand Field

Excited States: Their Roles in the Design and Performance of

Transition Metal Based Photochemical Molecular Devices. Coord.

Chem. Rev. 2011, 255, 591−616.

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.9b00229.

■

(9) Peng, Y.; Hu, J. X.; Lu, H.; Wilson, R. M.; Motevalli, M.;

S

́

Hernandez, I.; Gillin, W. P.; Wyatt, P. B.; Ye, H. Q. Functionalisation

of Ligands through Click Chemistry: Long-Lived NIR Emission from

Organic Er(III) Complexes with a Perfluorinated Core and a

Hydrogen-Containing Shell. RSC Adv. 2017, 7, 128−131.

(10) Tan, R. H. C.; Motevalli, M.; Abrahams, I.; Wyatt, P. B.; Gillin,

W. P. Quenching of IR Luminescence of Erbium, Neodymium, and

Ytterbium β-Diketonate Complexes by Ligand C−H and C−D

Bonds. J. Phys. Chem. B 2006, 110, 24476−24479.

NMR spectra of ligands and metal complexes; X-ray

diﬀraction details; crystallographic data and reﬁnement

parameters of complexes 2−7; additional luminescence

ﬁgures; ball and stick models for the partial structure of

8; percent of max intensity vs temperature curves for

complexes 1−4 and 8 (PDF)

(11) Zampetti, A.; Minotto, A.; Squeo, B. M.; Gregoriou, V. G.;

Allard, S.; Scherf, U.; Chochos, C. L.; Cacialli, F. Highly Efficient

Solid-State Near-Infrared Organic Light-Emitting Diodes Incorporat-

ing A-D-A Dyes Based on α,β-Unsubstituted “BODIPY” Moieties. Sci.

Rep. 2017, 7, 1611−1618.

(12) Park, S.; Fukuda, K.; Wang, M.; Lee, C.; Yokota, T.; Jin, H.;

Jinno, H.; Kimura, H.; Zalar, P.; Matsuhisa, N.; Umeza, S.; Bazan, G.

C.; Someya, T. Ultraflexible Near-Infrared Organic Photodetectors for

Conformal Photoplethysmogram Sensors. Adv. Mater. 2018, 30,

1802359−1802367.

(13) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of

Instrumental Analysis, 6th ed.; Thomson Brooks Cole: Belmont, CA,

2007.

(14) Ford, P. C.; Cariati, E.; Bourassa, J. Photoluminescence

Properties of Multinuclear Copper(I) Compounds. Chem. Rev. 1999,

99, 3625−3648.

(15) Bai, S.-Q.; Ke, K. L.; Young, D. J.; Hor, T. S. A. Structure and

Photoluminescence of Cubane-like [Cu₄I₄] Cluster-Based 1D

Coordination Polymer Assembled with Bis(Triazole)Pyridine Ligand.

J. Organomet. Chem. 2017, 849, 137−141.

Accession Codes

CCDC 1882285−1882290 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.

AUTHOR INFORMATION

Corresponding Author

*E-mail: mrose@cm.utexas.edu.

■

ORCID

Michael J. Rose: 0000-0002-6960-6639

Notes

The authors declare no competing ﬁnancial interest.

(16) Hardt, H. D.; Pierre, A. Fluorescence Thermochromism of

Pyridine Copper Iodides and Copper Iodide. Z. Anorg. Allg. Chem.

1973, 402, 107−112.

(17) Perruchas, S.; Goff, X. F. L.; Maron, S.; Maurin, I.; Guillen, F.;

Garcia, A.; Gacoin, T.; Boilot, J. P. Mechanochromic and

ACKNOWLEDGMENTS

We acknowledge ﬁnancial support from the Robert A. Welch

Foundation (F-1822), Research Corporation for Scientiﬁc

■

N

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Thermochromic Luminescence of a Copper Iodide Cluster. J. Am.

Chem. Soc. 2010, 132, 10967−10969.

Steric and Electronic Rationale For Metal Deposition. Inorg. Chem.

2018, 57, 10364−10374.

(18) Hardt, H. D. Lumineszenzthermochromie, Ein Vergessenes

(39) Issleib, K.; Hamann, B.; Schmidt, L. Antimon-Organo-

Verbindungen. III. Tert. Butylsubstituierte Stibine. Z. Anorg. Allg.

Chem. 1965, 339, 298−303.

̈

Phanomen. Naturwissenschaften 1974, 61, 107−110.

(19) Benito, Q.; Le Goff, X. F.; Nocton, G.; Fargues, A.; Garcia, A.;

́

Berhault, A.; Kahlal, S.; Saillard, J. Y.; Martineau, C.; Trebosc, J.;

(40) Breunig, H. J.; Severengiz, T. Reduction of Butyl- and

Phenylantimony(III) Bromides and of Tert-Butylantimony(III)

Chlorides with Magnesium in the Presence of Trimethylchlorosilane:

Competition of Antimony-Antimony and Antimony-Silicon Coupling.

Z. Naturforsch., B: J. Chem. Sci. 1982, 37B, 395−400.

Gacoin, T.; Boilot, J.-P.; Perruchas, S. Geometry Flexibility of Copper

Iodide Clusters: Variability in Luminescence Thermochromism. Inorg.

Chem. 2015, 54, 4483−4494.

(20) Knorr, M.; Bonnot, A.; Lapprand, A.; Khatyr, A.; Strohmann,

C.; Kubicki, M. M.; Rousselin, Y.; Harvey, P. D. Reactivity of CuI and

CuBr toward Dialkyl Sulfides RSR: From Discrete Molecular Cu₄I₄S₄

and Cu₈I₈S₆Clusters to Luminescent Copper(I) Coordination

Polymers. Inorg. Chem. 2015, 54, 4076−4093.

́

(41) Tsiatouras, V.; Banti, C. N.; Grzeskiewicz, A. M.; Rossos, G.;

Kourkoumelis, N.; Kubicki, M.; Hadjikakou, S. K. Structural,

Photolysis and Biological Studies of Novel Mixed Metal Cu(I)-

Sb(III) Mixed Ligand Complexes. J. Photochem. Photobiol., B 2016,

163, 261−268.

(21) Churchill, M. R.; Youngs, W. J. The Unexpected Isolation of

the Cubane-like Isomer of Tetrameric (Triphenylarsine)Copper(I)

Iodide. Crystal Structure of [(AsPh₃)CuI]₄·C₆H₆. Inorg. Chem. 1979,

18, 1133−1138.

(22) Taylor, W. V.; Soto, U. H.; Lynch, V. M.; Rose, M. J.

Antimony-Supported Cu₄I₄Cuboid with Short Cu-Cu Bonds:

Structural Premise for Near-Infrared Thermoluminescence. Inorg.

Chem. 2016, 55, 3206−3208.

(23) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W.

M.; O’Shea, D. F. In Vitro Demonstration of the Heavy-Atom Effect

for Photodynamic Therapy. J. Am. Chem. Soc. 2004, 126, 10619−

10631.

(24) Bolton, O.; Lee, K.; Kim, H.-J.; Lin, K. Y.; Kim, J. Activating

Efficient Phosphorescence from Purely Organic Materials by Crystal

Design. Nat. Chem. 2011, 3, 205−210.

(25) Easley, C. J.; Mettry, M.; Moses, E. M.; Hooley, R. J.; Bardeen,

C. J. Boosting the Heavy Atom Effect by Cavitand Encapsulation:

Room Temperature Phosphorescence of Pyrene in the Presence of

Oxygen. J. Phys. Chem. A 2018, 122, 6578−6584.

(26) Rose, M. J.; Mascharak, P. K. Photosensitization of Ruthenium

Nitrosyls to Red Light with an Isoelectronic Series of Heavy-Atom

Chromophores: Experimental and Density Functional Theory Studies

on the Effects of O-, S- And Se-Substituted Coordinated Dyes. Inorg.

Chem. 2009, 48, 6904−6917.

(27) Dobbs, K. D.; Boggs, J. E.; Cowley, A. H. Trends in Structure

and Reactivity of Group 15 Alkyls, Alkylidenes, and Alkylidynes.

Chem. Phys. Lett. 1987, 141, 372−375.

(28) Paizanos, K.; Charalampou, D.; Kourkoumelis, N.;

Kalpogiannaki, D.; Hadjiarapoglou, L.; Spanopoulou, A.; Lazarou,

K.; Manos, M. J.; Tasiopoulos, A. J.; Kubicki, M.; Hadjikakou, S. K.

Synthesis and Structural Characterization of New Cu(I) Complexes

with the Antithyroid Drug 6-n-Propyl-Thiouracil. Study of the Cu(I)-

Catalyzed Intermolecular Cycloaddition of Iodonium Ylides toward

Benzo[b]furans with Pharmaceutical Implementations. Inorg. Chem.

2012, 51, 12248−12259.

(29) CrystalClear 1.40; Rigaku Americas Corporation: The

Woodlands, TX, 2008.

(30) Sheldrick, G. M. SHELXT - Integrated Space-Group and

Crystal-Structure Determination Research Papers. Acta Crystallogr.,

Sect. A: Found. Adv. 2015, A71, 3−8.

(31) Sheldrick, G. M. Crystal Structure Refinement with SHELXL.

Acta Crystallogr., Sect. C: Struct. Chem. 2015, C71, 3−8.

(32) Spek, A. L. Structure Validation in Chemical Crystallography.

Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, D65, 148−155.

(33) Farrugia, L. J. WinGX and ORTEP for Windows: An Update. J.

Appl. Crystallogr. 2012, 45, 849−854.

(34) SAINT V8.27B; Bruker AXS Inc: Madison, WI, 2012.

(35) Granovsky, A. A. Fireﬂy v. 7; http://classic.chem.msu.su/gran/

ﬁ re ﬂ y/index.html.

(36) Bode, B. M.; Gordon, M. S. MacMolPlt: A Graphical User

Interface for GAMESS. J. Mol. Graphics Modell. 1998, 16, 133−138.

(37) Bilbrey, J. A.; Kazez, A. H.; Locklin, J.; Allen, W. D. Exact

Ligand Cone Angles. J. Comput. Chem. 2013, 34, 1189−1197.

(38) Taylor, W. V.; Xie, Z.; Cool, N.; Shubert, S.; Rose, M. J.

Syntheses, Structures and Characterization of Nickel(II) Stibines:

(42) Bowmaker, G. A.; Effendy; Hart, R. D.; Kildea, J. D.; Skelton, B.

W.; Silva, E. N. de; White, A. H. Lewis-Base Adducts of Group 11

Metal(I) Compounds. LXIV Syntheses, Spectroscopy and Structures

of Some 1 : 4 Adducts of Copper(I) and Silver(I) Perchlorates with

Triphenylarsine and Triphenylstibine. Aust. J. Chem. 1997, 50, 539−

552.

̈

(43) Wieber, M.; Hohl, H. Triorganoantimony Compounds as

Ligands in Iron Complexes. Z. Naturforsch., B: J. Chem. Sci. 1989, 44,

1149−1150.

(44) Breunig, H. J.; Lork, E.; Rat,̧ C. I.; Wagner, R. P. Syntheses and

Crystal Structures of [^tBu₃SbCr(CO)₅], [^tBu₃BiM(CO)₅] (M = Cr,

W), and [^tBu₃BiMnCp′(CO)₂] (Cp′=η⁵-C₅H₄CH₃). J. Organomet.

Chem. 2007, 692, 3430−3434.

(45) Huitorel, B.; El Moll, H.; Utrera-Melero, R.; Cordier, M.;

Fargues, A.; Garcia, A.; Massuyeau, F.; Martineau-Corcos, C.; Fayon,

F.; Rakhmatullin, A.; Kahlal, S.; Saillard, J.-Y.; Gacoin, T.; Perruchas,

S. Evaluation of Ligands Effect on the Photophysical Properties of

Copper Iodide Clusters. Inorg. Chem. 2018, 57, 4328−4339.

(46) Benito, Q.; Maurin, I.; Cheisson, T.; Nocton, G.; Fargues, A.;

Garcia, A.; Martineau, C.; Gacoin, T.; Boilot, J. P.; Perruchas, S.

Mechanochromic Luminescence of Copper Iodide Clusters. Chem. -

Eur. J. 2015, 21, 5892−5897.

(47) Tard, C.; Perruchas, S.; Maron, S.; Le Goff, X. F.; Guillen, F.;

Garcia, A.; Vigneron, J.; Etcheberry, A.; Gacoin, T.; Boilot, J. P.

Thermochromic Luminescence of Sol-Gel Films Based on Copper

Iodide Clusters. Chem. Mater. 2008, 20, 7010−7016.

(48) Lapprand, A.; Dutartre, M.; Khiri, N.; Levert, E.; Fortin, D.;

́

Rousselin, Y.; Soldera, A.; Juge, S.; Harvey, P. D. Luminescent P-

Chirogenic Copper Clusters. Inorg. Chem. 2013, 52, 7958−7967.

(49) Perruchas, S.; Tard, C.; Le Goff, X. F.; Fargues, A.; Garcia, A.;

Kahlal, S.; Saillard, J.-Y.; Gacoin, T.; Boilot, J.-P. Thermochromic

Luminescence of Copper Iodide Clusters: The Case of Phosphine

Ligands. Inorg. Chem. 2011, 50, 10682−10692.

(50) Churchill, M. R.; Kalra, K. L. Molecules with an M4 × 4 Core.

III. Comparison of the X-Ray Crystallographically Determined

Molecular Structures of Tetrameric Triethylphosphinecopper(I)

Iodide and Triethylarsinecopper(I) Iodide. Inorg. Chem. 1974, 13,

1899−1904.

(51) Medina, I.; Mague, J. T.; Fink, M. J. Tetra-μ₃-iodo-tetrakis[(tri-

tert-butylphosphine)Copper(I)]. Acta Crystallogr., Sect. E: Struct. Rep.

Online 2005, E61, m1550−m1552.

(52) Sculfort, S.; Braunstein, P. Intramolecular d¹⁰-d¹⁰Interactions

in Heterometallic Clusters of the Transition Metals. Chem. Soc. Rev.

2011, 40, 2741−2760.

(53) Gruttner, G.; Wiernik, M. Organic Antimony Compounds. II.

Preparation of Mixed Alkylarylstibines. Ber. Dtsch. Chem. Ges. 1915,

48, 1759−1764.

(54) Breunig, H. J.; Kanig, W. Preparation and Spectroscopic Studies

of Methyl and Isopropyl Halides. Phosphorus Sulfur Relat. Elem. 1982,

12, 149−159.

(55) Clavier, H.; Nolan, S. P. Percent Buried Volume for Phosphine

and N-Heterocyclic Carbene Ligands: Steric Properties in Organo-

metallic Chemistry. Chem. Commun. 2010, 46, 841−861.

O

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

(56) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.;

Oliva, R.; Scarano, V.; Cavallo, L. SambVca 2. A Web Tool for

Analyzing Catalytic Pockets with Topographic Steric Maps. Organo-

metallics 2016, 35, 2286−2293.

Intermolecular Hydrogen Bonds in Cu₄I₄Clusters in the Solid

State. Dalt. Trans. 2014, 43, 9448−9455.

(75) Tolman, C. A. Electron Donor-Acceptor Properties of

Phosphorus Ligands. Substituent Additivity. J. Am. Chem. Soc. 1970,

92, 2953−2956.

(57) Tolman, C. A. Steric Effects of Phosphorus Ligands in

Organometallic Chemistry and Homogeneous Catalysis. Chem. Rev.

1977, 77, 313−348.

(76) Chantzis, A.; Kowalska, J. K.; Maganas, D.; DeBeer, S.; Neese,

F. Ab Initio Wave Function-Based Determination of Element Specific

Shifts for the Efficient Calculation of X-Ray Absorption Spectra of

Main Group Elements and First Row Transition Metals. J. Chem.

Theory Comput. 2018, 14, 3686−3702.

(58) Imyanitov, N. S. Cone Angle of Ligands - Group IV and V

Compounds. Koord. Khim. 1985, 11, 1171−1178.

(59) Shou, R.-E.; Chai, W.-X.; Song, L.; Qin, L.-S.; Shi, H.-S.; Wang,

T.-G. Three Luminescent Copper(I) Iodide Clusters with Phosphine

Ligands: Synthesis, Structure Characterization, Properties and TD-

DFT Calculations. J. Cluster Sci. 2017, 28, 2185−2203.

(60) Bowmaker, G. A.; Effendy; Hart, R. D.; Kildea, J. D.; White, A.

H. Lewis-Base Adducts of Group 11 Metal(I) Compounds. LXXIII

Synthesis, Spectroscopy and Structural Systematics of New 1:1

’Cubane’ Tetramers of Copper(I) and Silver(I) Halides with

Triphenylarsine. Aust. J. Chem. 1997, 50, 653−670.

(61) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;

Springer: New York, 2006.

(62) Steiner, U. E. Fundamentals of Photophysics, Photochemistry,

and Photobiology. In Photodynamic Therapy; Abdel-Kader, M. H.,

Ed.; Springer-Verlag: Berlin, 2014; pp 25−58.

(63) Hardt, H. D.; Pierre, A. Fluorescence Thermochromism and

Symmetry of Copper(I) Complexes. Inorg. Chim. Acta 1977, 25,

L59−L60.

(64) Rath, N. P.; Holt, E. M.; Tanimura, K. Fluorescent Copper(I)

Complexes: Structural and Spectroscopic Characterization of Bis(p-

Toluidine)Bis(Acetonitrile)Tetraiodotetracopper and Bis[(p-

Chloroaniline)(Acetonitrile)Diiododicopper] Tetrameric Complexes

of Mixed-Ligand Character. Inorg. Chem. 1985, 24, 3934−3938.

(65) Hu, G.; Mains, G. J.; Holt, E. M. Correlation of Structure and

E m i s s i o n i n S o l i d S t a t e C o p p e r ( I ) C o m p l e x e s ;

[Cu₄I₄(CH₃CN)₂(L)₂], L = Aniline Derivative. Inorg. Chim. Acta

1995, 240, 559−565.

(66) Vogler, A.; Kunkely, H. Photoluminescence of Tetrameric

Copper(I) Iodide Complexes Solutions. J. Am. Chem. Soc. 1986, 108,

7211−7212.

(67) Yang, K.; Li, S.-L.; Zhang, F.-Q.; Zhang, X.-M. Simultaneous

Luminescent Thermochromism, Vapochromism, Solvatochromism,

and Mechanochromism in a C3-Symmetric Cubane [Cu₄I₄P₄] Cluster

without Cu−Cu Interaction. Inorg. Chem. 2016, 55, 7323−7325.

(68) Vega, A.; Saillard, J. Y. Bonding in Tetrahedral Cu₄(μ₃-X)₄L₄

Copper(I) Clusters: A DFT Investigation. Inorg. Chem. 2004, 43,

4012−4018.

(69) De Angelis, F.; Fantacci, S.; Sgamellotti, A.; Cariati, E.; Ugo, R.;

Ford, P. C. Electronic Transitions Involved in the Absorption

Spectrum and Dual Luminescence of Tetranuclear Cubane

[Cu₄I₄(Pyridine)₄] Cluster: A Density Functional Theory/Time-

Dependent Density Functional Theory Investigation. Inorg. Chem.

2006, 45, 10576−10584.

(70) Kitagawa, H.; Ozawa, Y.; Toriumi, K. Flexibility of Cubane-like

Cu₄I₄Framework: Temperature Dependence of Molecular Structure

and Luminescence Thermochromism of [Cu₄I₄(PPh₃)₄] in Two

Polymorphic Crystalline States. Chem. Commun. 2010, 46, 6302−

6304.

(71) Kim, T. H.; Shin, Y. W.; Jung, J. H.; Kim, J. S.; Kim, J. Crystal-

to-Crystal Transformation between Three Cu^ICoordination Poly-

mers and Structural Evidence for Luminescence Thermochromism.

Angew. Chem., Int. Ed. 2008, 47, 685−688.

(72) Ryu, C. K.; Vitale, M.; Ford, P. C. Photoluminescence

Properties of the Structurally Analogous Tetranuclear Copper(I)

Clusters Cu₄X₄(dpmp)₄(X = I, Br, Cl; dpmp = 2-(Diphenylmethyl)-

Pyridine. Inorg. Chem. 1993, 32, 869−874.

(73) Chen, K.; Shearer, J.; Catalano, V. J. Subtle Modulation of

Cu₄X₄L₂Phosphine Cluster Cores Leads to Changes in Lumines-

cence. Inorg. Chem. 2015, 54, 6245−6256.

(74) Mazzeo, P. P.; Maini, L.; Petrolati, A.; Fattori, V.; Shankland,

K.; Braga, D. Phosphorescence Quantum Yield Enhanced by

P

DOI: 10.1021/acs.inorgchem.9b00229

Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

Thermoluminescent Antimony-Supported Copper-Iodo Cuboids: Approaching NIR Emission via High Crystallographic Symmetry

DOI: 10.1021/acs.inorgchem.9b00229

Source and publish data:

Authors:

Article abstract of DOI:10.1021/acs.inorgchem.9b00229

Full text of DOI:10.1021/acs.inorgchem.9b00229

Products guided by the article

R&D Labs maybe for 17585-86-3

Relevant to this article

Hot Product