Luminescence color alteration induced by trapped solvent molecules in crystals of tetrahedral gold(i) complexes: Near-unity luminescence mixed with thermally activated delayed fluorescence and phosphorescence

Article abstract of DOI:10.1039/c9dt01373c

The tetrahedral gold(i) complex [Au(LtBu)2]Cl 1Cl [LtBu = 1,2-bis[bis(4-tert-butylphenyl)phosphino]benzene], having eight tert-butyl (tBu) groups, was synthesized and characterized. Emission color alteration caused by solvent molecules captured in the crystal lattice of 1Cl was investigated. The recrystallization of 1Cl in mixed solvents of tetrahydrofuran, THF, and various alkanes afforded two single crystals: 1Cl·THF (1BG), presenting intense blue-green luminescence with an emission peak wavelength, λmax, = 507 nm and 1Cl·THF·0.5n-hexane·H2O (1OR), exhibiting weak orange luminescence with λmax = 625 nm. The emission quantum yields, ΦPL, of the two complexes were 0.95 and 0.06, respectively. X-ray structural analysis revealed that all the solvent molecules captured in the crystal were located inside the spaces surrounded by the tBu groups of the 1⁺ cation. Further, an n-hexane molecule in 1OR was found to be sandwiched between two 1⁺ cations via CH/π interactions. The structure of 1Cl in 1OR could be distorted by the n-hexane molecule incorporated in the crystal, leading to the red-shift of the emission peak wavelength and the low quantum efficiency of 1OR over that of 1BG. On the other hand, [Au(L)2]Cl [L = 1,2-bis(diphenylphosphino)benzene], having no peripheral tBu groups, was unable to incorporate the solvent molecules in the crystal by recrystallization from the mixed solvents, and thus, the crystal solely yielded intense blue light emission. The results revealed that the tBu groups of the peripheral phenyl units are essential for the luminescence color alteration caused by the incorporation of organic solvent molecules into the crystal lattice. Finally, studies on the emission spectra and quantum yields in the temperature range 77-293 K revealed that luminescence from both 1BG and 1OR was composed of phosphorescence and thermally activated delayed fluorescence.

Full text of DOI:10.1039/c9dt01373c

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Luminescence color alteration induced by trapped solvent molecules in
crystals of tetrahedral gold(I) complexes: Near-unity luminescence mixed
with thermally activated delayed fluorescence and phosphorescence†
Masahisa Osawa,*a Hiroto Yamayoshi,a Mikio Hoshino,a Yuya Tanaka,b and Munetaka
Akitab
Abstract
The
tetrahedral
gold(I)
complex
[Au(LtBu)2]Cl
1Cl
[LtBu
=
1,2-bis[bis(4-tert-
butylphenyl)phosphino]benzene], having eight tert-butyl (tBu) groups, was synthesized and characterized.
Emission color alteration caused by solvent molecules captured in the crystal lattice of 1Cl was investigated.
The recrystallization of 1Cl in mixed solvents of tetrahydrofuran, THF, and various alkanes afforded two
single crystals: 1Cl▪THF (1BG), presenting intense blue-green luminescence with an emission peak
wavelength, max, = 507 nm and 1Cl•THF•0.5xn-hexane•H2O (1OR), exhibiting weak orange luminescence
with max = 625 nm. The emission quantum yields, PL, of the two complexes were 0.95 and 0.06,
respectively. X-ray structural analysis revealed that all the solvent molecules captured in the crystal were
located inside the spaces surrounded by the tBu groups of the 1+ cation. Further, an n-hexane molecule in
1OR was found to be sandwiched between two 1+ cations via CH/ interactions. The structure of 1Cl in 1OR
could be distorted by the n-hexane molecule incorporated in the crystal, leading to the red-shift of the
emission peak wavelength and the low quantum efficiency of 1OR over that of 1BG. On the other hand,
[Au(L)2]Cl [L = 1,2-bis(diphenylphosphino)benzene], having no peripheral tBu groups, was unable to
incorporate the solvent molecules in the crystal by recrystallization from the mixed solvents, and thus, the
crystal solely yielded intense blue light emission. The results revealed that the tBu groups of the peripheral
phenyl units are essential for the luminescence color alteration caused by the incorporation of organic solvent
molecules into the crystal lattice. Finally, studies on the emission spectra and quantum yields in the
temperature range 77–293 K revealed that luminescence from both 1BG and 1OR was composed of
phosphorescence and thermally activated delayed fluorescence.
1

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Introduction
Over the past decade, luminescence color alternation of metal complexes in the solid state induced by various
external factors has been the subject of extensive studies.1,2 In general, when excited, the molecule in fluid
solutions freely undergoes thermal relaxation within its lifetime, giving rise to emission from the thermally
relaxed excited state. However, because of the high rigidity of the surroundings, the excited molecule in the
crystals is allowed to undergo thermal relaxation solely by minimum molecular motion. Thus, the structure
changes between the ground and relaxed excited state in the crystals are regarded as being smaller than those
in fluid solutions. Accordingly, the luminescence spectrum of the molecule in a crystalline solid is
significantly dominated by the ground state structure.
In luminescence vapochromism, the incorporation of volatile organic compounds (VOCs) into crystals
transforms the ground state structure of the luminescent complexes and thus, luminescence from the crystals
is markedly changed by the incorporation of VOCs.3,4 Similarly, the molecular structures of the luminescent
complexes in crystals are modified by intra- and/or intermolecular weak interactions (e.g., hydrogen bond,
–, and CH/ interactions),5 metal–metal bond formation,6,7 and isomerization.8 Because of the variety of
emissions from these complexes, much effort is currently being made to understand the origin of the
luminescence color alternation observed in metal complexes on the basis of their molecular structures.
In our previous studies,9,10 we described luminescence color alteration by changing courter anions, X, of
crystalline tetrahedral gold(I) complexes, [Au(L)2]X, with the rigid diphosphine ligand [L = 1,2-
bis(diphenylphosphino)benzene]. Scheme 1A presents the structure of [Au(L)2]X. The changes in the
luminescence color are induced by the presence of the counter anion, which affects the molecular structure
of [Au(L)2]+ in the crystalline state. Particularly, the volume of the anion intensely affects the molecular
symmetry of the [Au(L)2]+ ions in the crystals. Thus, the relatively small anions, e.g., Cl– (packing view in
Scheme 1B), Br–, and I–, hardly affect the geometry of [Au(L)2]+. As a result, these complexes emit intense
–
blue emission regardless of the halogen anion present. However, the large anions, namely PF6 and B(C6H4F-
–
4)4 , induce the conformational change of the [Au(L)2]+ species in the crystals to a less tetrahedral geometry,
resulting in the large red-shift of the emission.9-11
According to the density functional theory (DFT) calculation, the HOMO and (HOMO-1) of an [Au(L)2]+
cation with perfect tetrahedral symmetry are degenerated and thus, the energies of the two orbitals are
identical. With an increase in distortion from this perfect tetrahedral symmetry, the energy difference, ED,
between the HOMO and (HOMO-1) of the Au(I) complexes increases and is accompanied by a decrease in
the energy gap between the HOMO and LUMO.9 These results imply that the increase in distortion from the
tetrahedral structure is equivalent to the increase in ED, leading to the red-shift in the emission from
[Au(L)2]+. Therefore, ED is regarded as an index for the evaluation of the distortion from tetrahedral
2

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symmetry. For example, for [Au(L)2]+, the ED of the blue-light-emitting form is ˂0.1 eV, while that of the
orange-light emitting form is >0.5 eV.9
As illustrated in Scheme 1C, the intraligand – interactions between two phenyl groups of ligand L are
essential to stabilize the symmetrical tetrahedral conformation of [Au(L)2]+, which emits in the blue region.
Since this interaction reduces the distance between two P atoms in L, the bite angle (P−Au−P) becomes small
[81.36(1)°]. We thus considered that the small bite angle is the evidence of the intraligand  interactions
between the two phenyl groups of L in [Au(L)2]+.
In this work, we have studied the luminescence color changes of Au(I) complexes caused by organic
solvent molecules captured in the crystal lattice. To capture the solvent molecules, it is necessary for the
complex to create space for the guest solvent molecule in the crystal lattice (Scheme 1B). Thus, the
introduction of large substituents in L is assumed to create space for the organic molecules in the crystal
12
lattice. In line with this assumption, 1,2-bis[bis(4-tert-butylphenyl)phosphino]benzene (LtBu
)
was
synthesized by the introduction of a tert-butyl (tBu) group at the para-position of each peripheral phenyl
group in 1,2-bis(diphenylphosphino)benzene (Scheme 1D).13-15 The intraligand – interactions between the
phenyl groups in LtBu become weaker than those in L. The tetrahedral Au(I) complex with the small anion
Cl–, [Au(LtBu)2]Cl (1Cl), was readily synthesized, using LtBu, in good yield. The tBu groups in 1Cl create
sufficient space in the crystal lattice for the guest solvent molecule to occupy. Photophysical studies in the
temperature range 293–77 K were carried out to elucidate the origin of the emission from crystalline 1Cl
obtained by recrystallization from various organic solvents.
(A)
(B)
Cl-
P
P
P
P
+
Au
X-
[Au(L)2]X
X-: various anions
3 Å
(C)
(D)
- interaction
 
intraligand
tBu
3.770(1) Å
centroid
tBu
centroid
P
P
P
P
Au
81.36(1)º
P
P
tBu
tBu
LtBu
two ligands are symmetric
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Scheme 1 (A) Molecular structure of [Au(L)2]X [L = 1,2-bis(diphenylphosphino)benzene]. (B) Packing view
(010) of [Au(L)2]Cl in capped-stick style and an intermolecular space (red dotted circle). (C) Intense blue
light-emitting form of [Au(L)2]+ cation in capped-stick style; hydrogen atoms have been omitted for clarity.
(D) Molecular structure of LtBu
.
Results and Discussion
Absorption Spectra in solution and related theoretical studies
Fig. 1 illustrates the absorption spectra of L, LtBu, [Au(L)2]Cl, and 1Cl in dichloromethane (CH2Cl2) at 293
K. The absorption spectrum of LtBu, having the peak wavelength at 379 nm, is very similar to that of L. The
molar absorption coefficient () at 379 nm was determined as 15,000 M-1cm-1 for L14 and 17,000 M-1cm-1 for
LtBu
.
The [Au(L)2]Cl complex in CH2Cl2 presented two absorption peaks at 280 and 295 nm with  = 31,000 M-
1cm-1, while 1Cl displayed an absorption shoulder at 280 nm with  = 40,000 M-1cm-1. Solutions containing
[Au(L)2]Cl and 1Cl were yellow due to absorption in the long wavelength region (>350 nm). According to
the DFT calculation, the long wavelength absorption can be ascribed to the electronic transition,  → *,
from the  orbital composed of Au d-orbitals and phosphine p-orbitals to the * orbitals of the ligands.9,11
Notably, although the crystals of both [Au(L)2]Cl and 1Cl are colorless, the solutions are yellow. This color
difference was elucidated on the basis of the DFT calculations.
Natural transition orbitals (NTO) analysis was carried out to assign the absorption bands. Figs. S1 and S2
display the optimized S0 structures and index values (ED) for an evolution of tetrahedral symmetry of 1+
and [Au(L)2]+ in CH2Cl2, which were obtained from DFT calculations. The index values are a measure of the
distortion from perfect tetrahedral symmetry. Both complexes at the S0 state in CH2Cl2 displayed a large
index, 0.54 eV. Since this ED was much larger than that calculated from the structure of [Au(L)2] in the
crystals (<0.1 eV), we concluded that compared to those in solution, the complexes in the crystals were more
likely to adopt structures close to tetrahedral symmetry.
Tables S1–S12 list the absorption maxima, oscillator strengths, NTO pairs, generation probability, and
characters for selected transitions. We initially performed the calculations of [Au(L)2]Cl using the atomic
coordinates determined by X-ray structure analysis and then optimized the S0 structure in CH2Cl2. The atomic
coordinates of the crystals reveal that the structure was markedly affected by the intraligand – interactions
(Scheme 1C). However, the optimized structure seem to be free from intraligand interactions (Fig. S1).
Accordingly, the results from the calculations obtained before optimization could be used to elucidate the
properties of the molecules in the crystals, while those calculated after optimization could be used for the
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molecules in the CH2Cl2 solutions.
For [Au(L)2]Cl, the S1←S0 transition energy before optimization was determined as 3.55 eV (349 nm)
(Table S17). After optimization of the structure, this value was reduced to 3.32 eV (374 nm). Clearly, the
S1←S0 transition shifted to a lower energy after optimization, in agreement with the observation that the
colorless [Au(L)2]Cl crystals become yellow when dissolved in CH2Cl2. This explanation could also be
applied to 1Cl.
The oscillator strengths of the Sn←S0 (n ≤ 40) transitions of [Au(L)2]+ and 1+ in CH2Cl2 are presented in
the inset of Fig. 1. The main transition for the lower-lying state (S1←S0) of the 1+ and [Au(L)2]+ ions was
established as  → * (the bridging phenylene unit): the S1←S0 is governed by the HOMO-LUMO transition.
Both complexes possess similar transitions in the UV-vis region; however, 1Cl exhibits intense absorption
at ~255 nm (S40←S0; inset of Fig. 1). The introduction of the strongly electron-donating group (tBu) rises
the energy level of the HOMO () and thus, the –* bands in the absorption spectrum of 1Cl were expected
to be red-shifted compared to those of [Au(L)2]Cl. However, the absorption spectrum of 1Cl was located at
wavelengths shorter than that of [Au(L)2]Cl. This suggested that the S40 transition (S40←S0), having a large
oscillator strength assigned to –* (transitions from a p-tBu-phenylene ring to an o-phenylene ring) uplifts
the absorption spectrum at ~250 nm, resulting in an apparent slight blue-shift in the maximum emission
wavelength (max) of 1Cl over that of [Au(L)2]Cl.
As in the case of [Au(L)2]X, 1Cl did not present any detectable emission in the solutions because of the
“pseudo-Jahn-Teller effect”. 11,14b The NTO analysis carried out for 1+ demonstrated that a structural change
occurs from the tetrahedral to the flattened form at the T1 optimized geometry. The dihedral angle between
the two planes (P1–Au1–P2 and P3–Au1–P4) is as small as 38.02° and the ED value is 1.90 eV for T1 in
CH2Cl2, indicating that the molecular structure of T1 is more flattened than that of S0 (Fig. S3). Because these
two values (dihedral angle and ED) of 1+ are similar to those of [Au(L)2]+,11 we concluded that the bulky
tBu groups in 1+ were unable to prevent the structural changes, leading to the absence of emission in the
solutions. As will be discussed later, crystalline [Au(L)2]Cl and 1Cl exhibited both phosphorescence and
thermally activated delayed fluorescence (TADF). The T1 state responsible for emission is composed of
metal-centered (3MC) and ligand-to-metal charge-transfer (3LMCT: from the P atoms to the metal) states
(Tables S13 and S14).
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5.0
4.0
3.0
2.0
1.0
0.0
0.25
0.20
0.15
0.10
0.05
0.00

260 280 300 320 340 360
wavelength / nm
250
300
350
400
450
500
Wavelength / nm
Fig. 1 Absorption spectra of L (red dots), LtBu (blue dots), [Au(L)2]Cl (red), and [Au(LtBu)2]Cl 1 (blue) in
CH2Cl2 at 293 K. Inset: oscillator strengths of the calculated Sn←S0 (n ≤40) transitions at the optimized S0
geometry of [Au(L)2]+ (red) and 1+ (blue) in CH2Cl2.
Structures of the crystals obtained from a solvent mixture of THF and alkanes
The complex 1Cl was recrystallized from the THF solution by the addition of poor solvents (alkanes). n-
Pentane (1.7 mL) was slowly added to a yellow THF solution (1 mL) of 1Cl (12 mg, 7.6 x 10-3 mmol) in a
test tube by a micro-syringe at 293 K and the tube was immediately closed with a lid. After 12 h, the solution
turned from yellow to colorless, generating colorless crystals of 1Cl▪THF (1BG) (Fig. 2A). This 1BG
complex emitted intense blue-green light at 293 K with a max of 507 nm and a quantum yield (PL) of 0.95.
The molecular structure of 1BG determined by X-ray analysis is presented in Fig. 2B, while the
crystallographic data and selected bond lengths and angles are summarized in Tables S15 and S16,
respectively. The Au−P bond lengths (2.37−2.44 Å) of 1BG were similar to those of tetrahedral [Au(L)2]Cl.9
However, the two bisphosphine ligands in 1BG asymmetrically coordinated to the Au center. Because of the
relatively short distance [4.168(2) Å] between the centroids of the two 4-tBu-phenylene groups (Fig. 2B), we
assumed that one of the two bis-phosphine ligands displayed a weak intraligand − interaction between the
two 4-tBu-phenylene groups.
The increase in ED, the emission peak wavelengths from the Au(I) complexes shift to red and the emission
yields decrease. 9 For example, the complex having the ED of 0.1 eV presented intense blue emission, while
that with an ED of 0.5 eV displayed weak yellow-orange emission. The ED, 0.37 eV, calculated for 1BG
in the present study is just in the range 0.1−0.5 eV, in accordance with the intense blue-green emission of
crystalline 1BG (Fig. 2C).
In the crystal lattice, one THF molecule is trapped by each 1Cl molecule. Fig. 2D displays the packing
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view (010) of 1BG; the THF molecules are located in the intermolecular space surrounded by the t-Bu groups
of 1+.
(A)
(B)
Cl
P2
•
centroid-centroid
distance: 4.178(2) Å
P3
P1
Au1
•
P4
THF
(D)
(C)
P4
P2
Au1
86.47 (4) °
P1
80.66 (4) °
P3
83.88 (4) °
= 0.37 eV
ED
7.5 Å
Fig. 2 (A) Images of the yellow THF solution of 1Cl (left), colorless crystals of 1BG (center), and intense
blue-green emission of 1BG; exc =365 nm (right). (B) Crystal structure of 1BG in capped-stick style.
Hydrogen atoms and the disordered atoms in the tBu groups have been omitted. (C) Core structure of 1BG
and (D) packing view (010) of 1BG and an intermolecular space in which the THF molecules exist (red
dotted circle).
The recrystallization of 1Cl from the THF solutions was examined using alkanes [n-hexane (C6), n-
heptane (C7), n-octane (C8), n-nonane (C9), and n-decane (C10)] other than n-pentane (C5). Complex 1Cl
(72 mg, 4.6 x 10-2 mmol) was dissolved in THF (6.0 mL, 74 mmol). After membrane filtration, 1 mL yellow
THF solution was transferred to each of six test tubes. Each alkane (C5−C10, 1.3 mL, 6.4−11 mmol) was
slowly added to the THF solution of 1Cl in each test tube, at 293 K, and the tubes were then sealed with caps
(Fig. 3A). After 12 h, colorless crystals of 1BG were formed in the test tubes containing C5 and C7−C10
alkanes, while no crystal formation was observed for the yellow THF solution containing C6 (Figs. 3B and
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3C). Thus, a further 0.5 mL n-hexane was added to the yellow solution of the C6 tube, resulting in the
formation of yellow plate-like crystals of 1OR after 24 h (Fig. 4A).
Complex 1OR displayed weak orange luminescence with a max of 625 nm and PL of 0.06. Fig. 4B
illustrates the crystal structure of 1OR: one molecule of THF and a half molecule of n-hexane per one
molecule of 1Cl are captured in the yellow crystals. Crystallographic data and selected bond lengths and
angles are summarized in Tables S15 and S16, respectively. Because of the P21/n space group, the n-hexane
molecule is located at the inversion center. A weak interligand − interaction [centroid–centroid distance
between two 4-tBu-phenylene groups: 4.2332(3) Å] could be observed.
Fig. 4C presents a packing structure in which two 1+ cations sandwich the n-hexane molecule at both ends
via CH/ interactions and the THF molecule has an intermolecular CH/ interaction with one bridged
phenylene unit of LtBu. The distances between the hydrogen atom of the CH unit and the  system, 2.81 Å
(n-hexane−1+) and 2.87 Å (THF−1+), are in good agreement with those observed for other
CH/ interactions.16
The large ED of 0.73 eV and the dihedral angle of 69.29(4)° between two planes, P1−Au−P2 and
P3−Au−P4, suggested that the 1+ cation in the 1OR crystals adopts a highly distorted tetrahedral geometry.
This structural distortion destabilizes the HOMO energy and accelerates the non-radiative decay of the
emissive excited state. The ED of the 1OR crystals was much larger than that of 1BG and thus, (1) the
former crystals were yellow, owing to the destabilization of the HOMO energy, even though the latter ones
were colorless and (2) emission from the former crystals were redshifted and the emission quantum yield was
lower than that of the latter crystals.
This implies that the trapped n-hexane molecule affects the structure of the 1+ cation through CH/
interactions. However, DFT calculation results indicate that the n-hexane molecules have a minimum
influence on the molecular orbitals of LUMO and HOMO which are responsible for emission from 1OR
(Table S17). Dramatic changes in the emission from single crystals caused by the weak CH/ interaction
have occasionally been reported in earlier papers.17
As previously mentioned, the THF solutions of 1Cl only afforded a colorless 1BG crystal when C5 and
C7−C10 alkanes were added, while yellow crystals of 1OR were obtained with n-hexane. In the 1OR crystals,
an n-hexane molecule is sandwiched between two I+ cations via CH/ interaction. We assumed that among
the alkanes studied, n-hexane exhibited the best molecular length and volume to be incorporated in the
crystals. The branched hexanes, 2-methylhexane and 3-methylhexane, were added to the THF solutions of
1Cl as poor solvents. However, these branched hexanes were not captured in the crystals and only colorless
crystals of 1BG were obtained.
Similarly, recrystallization of [Au(L)2]Cl was carried out in THF with the use of these alkanes as poor
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solvents. However, this solely afforded intense blue light-emitting crystals with two symmetrical diphosphine
ligands (Scheme 1C). No incorporation of THF and alkanes was observed, leading to the conclusion that the
tBu groups of the peripheral phenyl units in 1Cl are essential for emission color alterations by incorporation
of organic solvents into the crystal lattice.
(72 mg, 4.6X10-2 mmol)
1
in THF (6.0 mL)
1.0 mL
1.0 mL
1.0 mL
1.0 mL
1.0 mL
1.0 mL
(A)
n-nonane n-decane
n-octane
n-hexane n-heptane
n-pentane
1.3 mL
1.3 mL
1.3 mL
1.3 mL
1.3 mL
1.3 mL
(B)
(C)
Fig. 3 Recrystallization procedure with THF and alkanes (C5–C10): (A) image of the solutions just after
alkane addition; (B) image of the solutions after 12 h; and (C) emission images from 1BG; exc = 365 nm.
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(A)
(B)
interligand
- interaction
 
4.2332(3) Å
•
•
P4
P4
P2
86.58(4)°
P2
Au1
P1
Au1
85.10(4)°
P3
P3
P1
THF
Cl
69.29(6)°
= 0.73 eV
ED
n-hexane
(Cii)
(Ci)
CH/ interaction

2.81(2) Å
n-hexane
2.81(2) Å
(Ciii)
CH/ interaction

2.87(2) Å
THF
THF
2.87(2) Å
Fig. 4 (A) Image of crystals of 1OR (left) and luminescence image of 1OR (right); exc = 365 nm. (B) Crystal
structure and the core structure of 1OR in capped-stick style. Hydrogen atoms, disordered atoms in the tBu
groups, and oxygen atoms in water have been omitted for clarity. (Ci) Packing view (100) of 1OR. (Cii)
Magnified view of the CH/ interactions (red lines) between n-hexane and 1+ (two THF molecules and two
4-tert-butylphenyl groups are omitted for clarity). (Ciii) Magnified view of the CH/ interactions (red lines)
between THF and 1+.
Structure of crystals obtained from a solvent mixture of CH2Cl2 and alkanes
Dichloromethane (CH2Cl2), instead of THF, was used as a good solvent. Complex 1Cl (12 mg, 7.6 x 10-3
mmol) was dissolved in CH2Cl2 (0.5 mL, 7.8 mmol). The alkane (C6–C10, 1.3 mL, 6.4–10 mmol) was slowly
added to the 0.5 mL yellow CH2Cl2 solution and the tubes were then sealed with a perforated cap at 293 K.
Crystallization was successfully carried out by the evaporation of CH2Cl2 at room temperature via a
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perforated cap. After 12 h, pale yellow crystals of 1Cl•1.5xCH2Cl2•2.5xH2O (1YL) were formed,
independent of the alkane used (Fig. 5A). This complex emitted yellow light at 293 K with a max of 580 nm
and PL of 0.23. The crystallographic data and selected bond lengths and angles are summarized in Tables
S15 and S16, respectively. Fig. 5B displays the crystal structure of 1YL: one very weak intraligand –
interaction [centroid–centroid distance = 4.669(4)] and one very weak interligand – interaction [centroid-
centroid distance = 4.437(4)] were observed. The dihedral angle [78.83(8)°] and ED (0.42 eV) of 1YL in
Fig. 5C were very similar to those (78.52° and 0.54 eV, respectively) of the optimized S0 structure of 1+ in
CH2Cl2 (Fig. S2). These values ranged between those of 1BG and 1OR. Thus, the above-mentioned emission
properties were well consistent with the structural features of 1YL. As in the cases of 1BG and 1OR, CH2Cl2,
H2O, and Cl- were located in the intermolecular space (Fig. 5D), which might be important to stabilize the
structure of 1YL.
(A)
CH2Cl2
(B)
0.5CH2Cl2
intraligand - interaction
•
P1
centroid-centroid
•
P4
distance: 4.669(4) Å
Au1
P3
P2
interligand - interaction
•
centroid-centroid distance: 4.437(4) Å
•
0.5Cl
0.5Cl
(D)
(C)
P4
P2
87.13(6)°
P1
Au1
82.83(5)°
P3
78.83(8) Å
ED = 0.42 eV
Fig. 5 (A) Image of the 1YL crystals (left) and luminescence image of 1YL (right); exc = 365 nm. (B) Crystal
structure of 1YL in capped-stick style. Hydrogen atoms, oxygen atoms in the water molecules, and disordered
atoms in the tBu groups have been omitted for clarity. (C) Core structure of 1YL and (D) Packing view (100)
of 1YL and intermolecular space (red dotted circle).
Photophysical properties of Au(I) complexes in crystal form
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The emission and excitation spectra of the crystals of 1BG, 1YL, and 1OR at 293 K are presented in Fig. 6
and the photophysical data of [Au(L)2]Cl, 1BG, 1YL, and 1OR at 293 and 77 K are summarized in Table 1.
All these gold complexes exhibit very broad and unstructured emission spectra. Emission energies of the
three complexes are in the order of 1BG > 1YL > 1OR as listed in Table 1. Taking account of the fact that
the dihedral angles are in the order of 1BG(83.88°) > 1YL(78.83°) > 1OR(69.29 °), the emission energy is
considered to be closely related to the degree of the distortion from the perfect tetrahedral symmetry. Since
the ED values, a measure of the distortion, are in the order of 1BG < 1YL < 1OR, the emission peak
wavelengths shift to red with an increase in the degree of the distortion. Emission from [Au(L)2]Cl, 1BG,
and 1OR decays according to first order kinetics in the temperature range 77–293 K. However, the decay of
the emission from 1YL is expressed as the sum of the two exponential functions of time. Thus, the two
emission lifetimes of 1YL obtained at 77 and 293 K are listed in Table 1. All the studied the complexes have
relatively short lifetimes (0.48–3.4 s) at 293 K. The lifetimes of the emission at 77 K are one order of
magnitude longer than those at 293 K. These observations suggest that room temperature emission can be
ascribed to TADF.18 Because of the small singlet (S1)-triplet (T1) energy gap, TADF occurs from S1, which
is thermally populated from T1 via the reverse-intersystem crossing process.
1BG 1YL
1OR
300
400
500
600
700
800
Wavelength / nm
Fig. 6 Corrected emission and excitation spectra of 1BG, 1YL, and 1OR crystals; exc = 355 nm, em = 507
nm for 1BG, 580 nm for 1YL, and 625 nm for 1OR.
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Table 1 Photophysical properties of the [Au(L)2]Cl, 1BG, 1OR, and 1YL crystals
293 K
77 K
E(S1-T1)
=1/k /sb
/ cm-1(eV)
720(0.089) d
380(0.047)d
–
max / nm(eV)a
485(2.556)
507(2.445)
580(2.138)
625(1.984)
 c
0.95
0.95
0.23
0.06

max / nm(eV)a
485(2.556)
510(2.431)
585(2.119)
650(1.907)
 c
0.95
0.95
0.90
0.28
=1/k / sb
[Au(L)2]Cl
1BG
3.4
3.3
18
48
1YL
1.3 [0.53(31), 1.62(69)]e
0.48
24 [4.3(27), 31(73)]e
10
1OR
320(0.040)d
a Emission maxima; exc = 355 nm. b Emission decay time; exc = 355 nm. c Absolute PL quantum yield (error ± 8%); exc = 355 nm.
d
e
Energy gap between the S1 and T1 levels obtained by temperature dependence of the decay time. Two-component. The
percentage given inside parentheses represents the composition ratio of the lifetime.
We next measured the decay rate constants, k, of emission as a function of temperature, T, in the
temperature range 77–293 K to determine the S-T energy gap [E(S1-T1)] for [Au(L)2]Cl, 1BG, and 1OR.
The temperature-dependent changes of the corrected emission spectra and the lifetimes are presented in Figs.
S4–S6.
The decay rate constant, k (=1/av), is defined by eq. (1):19
3kT + kS exp(-E(S1 - T1)/kBT)
=
k
3 + exp(-E(S1 - T1)/kBT)
(eq. 1)
where kB represents the Boltzmann constant and kS(=1/S) and kT(=1/T) denote the rate constants for the
decay of the singlet and the triplet state, respectively. Curve fitting analyses of the plots of kversus T obtained
for [Au(L)2]Cl, 1BG, and 1OR were carried out to determine the values of kT, kS, and E(S1-T1) using eq. 1
and the least-square method. The E(S1-T1) values of [Au(L)2]Cl, 1BG, and 1OR were determined as 720,
380, and 320 cm-1, respectively. The rate constants, kT, for these complexes were in the range 2 x 104 –1.00
x 105 s-1, while the kS values were in the range 6 x 106–2.4 x 107 s-1. These results suggest that [Au(L)2]Cl,
1BG, and 1OR afford emissions from both the triplet and the thermally activated singlet states. The E(S1-
T1) values of these complexes are listed in Table 1, while their ks and kT values are listed later in the
manuscript in Table 2.
Since the emission decay from 1YL was not expressed as a single exponential function of time, no detailed
analysis was carried out in the temperature range 77–293 K. However, emission from 1YL was assumed to
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be a mixture of phosphorescence and TADF since the emission peak wavelengths and lifetimes measured at
77 and 293 K are in agreement with those of 1BG and 1OR.
As mentioned above, the emissions of [Au(L)2]Cl, 1BG, and 1OR originate from the excited triplet, T1,
and thermally activated singlet, S1, states. The radiative rate constants at T1 and S1 are determined as follows:
From eq. 1, the radiative rate constant, k(r) for emission is:
3kT(r) + kS(r) exp(-E(S1 - T1)/kBT)
=
k(r)
3 + exp(-E(S1 - T1)/kBT)
(eq. 2)
where kT(r) and kS(r) are the radiative rate constants of T1 and S1, respectively.
From eqs. 1 and 2, the quantum yield ( is formulated as:
k(r)
3kT(r) + kS(r) exp(-E(S1 - T1)/kBT)
3kT + kS exp(-E(S1 - T1)/kBT)
=

=
k
(eq. 3)
The values of kS(r) and kT(r) are calculated from, kS, kT, E(S1-T1), and the emission quantum yields, ,
at 77 and 293 K. For [Au(L)2]Cl, the values kT (= 1/T) = 5.56 x 104, 293 = 0.95, and 77 = 0.95 and those
determined from the curve fitting E(S1-T1) = 720 cm-1, kS (= 1/S) = 2.42 x 107, led to eqs. 4 and 5:
3kT(r) + kS(r) x 2.97 X10-2
=
293
=
0.95
0.95
8.85 X105
(eq. 4)
3kT(r) + kS(r) x 1.54 X 10-8
=
77
=
1.67 X105
(eq. 5)
The radiative rate constants can be obtained with values of kS(r) = 2.30 x 107 and kT(r) = 5.28 x 104 by
applying eqs. 4 and 5.
The quantum yields of the TADF and phosphorescence, (TADF) and (Phos), respectively, can be
obtained from eqs. 6 and 7:
kS(r) exp(-E(S1 - T1)/kBT)
=
(TADF)


3kT + kS exp(-E(S1 - T1)/kBT)
(eq. 6)
3kT(r)
(Phos)
=
3kT + kS exp(-E(S1 - T1)/kBT)
(eq. 7)
Thus, the composition of 293(TADF) and 293(Phos) for [Au(L)2]Cl were calculated as 0.81 and 0.19,
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respectively, and that of 77(TADF) and (Phos) as 0.0 and 1.0, respectively. These results suggest that
the [Au(L)2]Cl emission is TADF containing 20% phosphorescence at 293 K and pure phosphorescence at
77 K. The rate constants and TADF-to-phosphorescence ratios for [Au(L)2]Cl, 1BG, and 1OR are presented
in Table 2. [Au(L)2]Cl, 1BG, and 1OR have similar kT(r) values (Table 2). Since kT(r) is governed by the
spin orbit interaction, all these complexes are assumed to have a magnitude similar to that of spin-orbit
coupling (SOC).
Table 2 Rate constants and TADF-to-phosphorescence ratios for [Au(L)2]Cl, 1BG, and 1OR.
(TADF) : (Phos)
ks(nr)a / s-1
1.20 X 106
2.90 X 105
2.17 X 107
kT(nr) b / s-1
2.80 X 103
1.00 X 103
6.75 X 104
kS(r) / s-1
2.30 X 107
5.59 X 106
9.80 X 105
kT(r) / s-1
5.28 X 104
1.79 X 104
3.25 X 104
293 K
81 : 19
94 : 6
77 K
0 : 100
8 : 92
2 : 98
kS (= 1/S) / s-1
2.42 X107
kT (= 1/T) / s-1
5.56 X 104
1.89 X 104
1.00 X 105
[Au(L)2]Cl
1BG
5.88 X 106
2.27 X 107
1OR
32 : 68
a kS(nr) = kS – kS(r), b kT(nr) = kT – kT(r)
The excited singlet and triplet states of these complexes reached thermal equilibrium in the studied
temperature. Thus, the emission quantum yield TADF-to-phosphorescence ratio, (TADF)/(Phos),
becomes large with an increase in the thermal population of the excited singlet state. It is therefore reasonable
to assume that 1BG, with a small E(S1-T1) value, displays larger (TADF)/(Phos) ratios than those of
[Au(L)2]Cl. However, 1OR, with the smallest E(S1-T1) among the studied complexes, presented the
smallest (TADF)/(Phos) ratios. This was attributed to the significantly larger kS(nr) value of 1OR over
those of [Au(L)2]Cl and 1BG due to the distorted structure in the crystal. Further, the smallest kS(r) value of
1OR is responsible for the low emission quantum yield of 0.06 at 293 K.
[Au(L)2]Cl, 1BG, and 1OR exhibit luminescence mixed with TADF and phosphorescence at 293 K,
presumably due to the enhancement of the SOC by the heavy atom effect of the Au atom. In particular,
[Au(L)2]Cl and 1BG exhibit near-unity blue or blue-green luminescence at both 293 and 77 K. There are
several reports on Cu(I) and Ag(I) complexes displaying efficient room-temperature emission mixed with
TADF and short-lived phosphorescence according to the effective SOC.20
Summary
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The Au(I) complex, [Au(LtBu)2]Cl 1Cl [LtBu = 1,2-bis[bis(4-tert-butylphenyl)phosphino]benzene], having
eight external tBu groups, was synthesized and characterized. Recrystallization of 1Cl in organic solvents
revealed that the solvent molecules were captured inside the intermolecular space surrounded by tBu groups
of 1Cl molecules in the crystal lattice. The THF solution of 1Cl was yellow, suggesting that the stable
structure in the ground state was markedly distorted from the tetrahedral symmetry in the fluid solutions. The
complex, 1BG, was considered to release molecular distortion by capturing THF in crystals and therefore,
the 1BG crystals were colorless and emitted intense blue-green emission with max = 507 nm and PL = 0.95.
On the other hand, because of the substantial distortion from tetrahedral symmetry, 1OR crystals comprising
THF, n-hexane, and water molecules were yellow and exhibited weak orange emission with max = 625 nm
and PL = 0.06. These results suggest that the molecular structure of 1Cl in crystals can be altered by
capturing the solvent molecule into an intermolecular space surrounded by tBu groups. Furthermore,
thermogravimetric analyses displayed in Fig. S11 reveal that the solvent molecules are stably trapped in the
space at room temperature with supramolecular interactions.
Luminescence studies carried out in the temperature range 77–293 K demonstrated that [Au(L)2]Cl, 1BG,
and 1OR emit TADF and phosphorescence at 293 K with phosphorescence ratios of ~20, 6, and 70%,
respectively.
Experimental
Materials
1,2-Bis(dichlorophosphino)benzene was purchased from FUJIFILM Wako Pure Chemical Co, while 4-tert-
butylmagnesium bromide and chloro(dimethylsulfide)Au(I) were purchased from Sigma-Aldrich Co. LLC.
All the recrystallization organic solvents (THF, CH2Cl2, n-pentane, n-hexane, n-heptane, n-octane, n-nonane,
n-decane, 2-methylhexane, and 3-methylhexane) were purchased from FUJIFILM Wako Pure Chemical Co.
and used as received without further purification.
1,2-bis[bis(4-tert-butylphenyl)phosphino]benzene (LtBu
)
A THF solution of 4-tert-butylphenyl magnesium bromide (0.5 M, 100 mL) was added dropwise to a THF
solution (20 mL) of 1,2-bis(dichlorophosphino)benzene (2.0 g, 7.14 mol) at 0 ºC. The reaction mixture was
stirred under reflux for 2 h and subsequently cooled to room temperature. A solution of saturated aqueous
NH4Cl (100 mL) was next added to the reaction mixture. The product was extracted with CH2Cl2 (3 x 60
mL) and the combined organic extracts were dried over MgSO4. The drying agent was removed by filtration
and the solvent was removed in vacuo to produce a pale yellow solid. The residue was purified by
chromatography on silica gel (n-hexane/CH2Cl2, 3:1) to afford LtBu, 3.51 g (74%) as white powder. 1H NMR
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(400 MHz, CD2Cl2, 300 K) δ 7.32 (m, 8H, p-tBu-phenylene), 7.25 (m, 2H, o-phenylene), 7.12 (m, 8H, p-
tBu-phenylene), 7.08 (m, 2H, o-phenylene), 1.31 (s, 36 H, tBu); 31P NMR (162 MHz, CD2Cl2) δ -15.34 (s).
Anal. Calcd. for C46H56P2: C, 82.35; H, 8.41; Found: C, 82.15; H, 8.29.
[Au(LtBu)2]Cl (1Cl)
A CH2Cl2 solution (10 mL) of Me2SAuCl (44 mg, 0.15 mmol) was added to a solution of LtBu (200 mg, 0.30
mmol) in CH2Cl2 (20 mL). The mixture was stirred for 3 h at room temperature. The yellow reaction mixture
was filtrated and the solvent was removed in vacuo to produce a yellow amorphous solid. The residue was
purified by recrystallization from diethyl ether under ultrasonic waves to produce colorless microcrystals.
Yield 221 mg, 94%. 1H NMR (400 MHz, CD2Cl2)  7.51 (m, 2H, o-phenylene), 7.40 (m, 2H, o-phenylene),
7.14 (m, 8H, p-tBu-phenylene), 7.01 (m, 8H, p-tBu-phenylene), 1.29 (s, 36 H, tBu); 31P NMR (162 MHz,
CD2Cl2) δ 18.14 (s). Anal. Calcd. for C92H112AuClP4: C, 70.19; H, 7.17; Found: C, 70.33; H, 7.39.
1BG (1Cl▪THF)
Colorless micro crystals of 1Cl (50 mg, 3.2X10-2 mmol) were dissolved in THF (2.0 mL). After membrane
filtration, 1.5 mL of n-pentane was slowly added to the yellow THF solution of 1Cl in test tube at 293 K, and
the tubes were then sealed with caps. After 24 h, colorless crystals of 1BG (37 mg, 2.3X10-2 mmol) were
formed in the test tubes. Isolated yield is 71% as single crystals. Anal. Calcd. for C96H120AuClOP4: C, 70.04;
H, 7.35; Found: C, 70.15; H, 7.26.
1OR (1Cl•THF•0.5xn-hexane•H2O)
Colorless micro crystals of 1Cl (50 mg, 3.2X10-2 mmol) were dispersed in n-hexane (5.0 mL). Then, 2 mL
of THF was slowly added to the n-hexane solution in test tube at 50 ℃. After standing for 48 h at 293 K,
yellow crystals of 1OR (24 mg, 1.4 X10-2 mmol) were formed in the test tubes. Isolated yield is 44% as
single crystals. Anal. Calcd. for C99H129AuClO2P4: C, 69.64; H, 7.62; Found: C, 69.47; H, 7.52.
1YL (1Cl•1.5xCH2Cl2•2.5xH2O)
Colorless micro crystals of 1Cl (50 mg, 3.2X10-2 mmol) were dissolved in CH2Cl2 (2.0mL). After membrane
filtration, 2.0 mL of n-hexane was added to the yellow CH2Cl2 solution at 293 K. After standing for 48 h,
pale yellow crystals of 1YL (22mg, 1.3 X 10-2) were obtained. Small yellow amorphous portion is observed.
Isolated yield is 41% as single crystals. Anal. Calcd. for C187H240Au2Cl8O5P8: C, 64.30; H, 6.93; Found: C,
64.72; H, .7.10.
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Sample preparations for photophysical measurements
Single crystals of 1BG, 1OR and 1YL were used for photphysical measurements. These single crystals are
stable at 293 K based on thermogravimetric analyses (Fig. S11).
General Information
Details are given in the ESI†.
Conflicts of interests
There are no conflicts of interest to declare.
Acknowledgements
M. O. is grateful to JSPS KAKENHI Grant Number JP19K05635. This work was partly performed under the
Cooperative Research Program of "Network Joint Research Center for Materials and Devices." Theoretical
calculations were partly performed at the Research Center for Computational Science, Okazaki, Japan.
Notes and References
a
Department of Applied Chemistry, Nippon Institute of Technology, Gakuendai 4-1, Miyashiro-Machi,
Saitama, 345-8501, Japan
b Laboratory for Chemistry and Life Science Institute of Innovative Research, Tokyo Institute of Technology
R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
† Electronic Supplementary Information (ESI) available: general information, crystallographic data, NMR
spectra, theoretical calculations, and crystallographic data of 1BG, 1OR, and 1YL; CCDC 1902631–
1902633. For ESI and crystallographic data in CIF or other electronic formats see DOI: 10.1039/X000000x
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For Table of Contents
Luminescence color alteration induced by trapped solvent molecules in crystals of tetrahedral gold(I)
complexes: Near-unity luminescence mixed with thermally activated delayed fluorescence and
phosphorescence†
Masahisa Osawa, Hiroto Yamayoshi, Mikio Hoshino, Yuya Tanaka, and Munetaka Akita
TADF + Phos.
THF
n-hexane
THF
THF
THF
SYNOPSIS
Emission color alteration caused by captured solvent molecules in the crystal lattice of tetrahedral gold(I)
complexes.
1