Experimental and theoretical studies of highly emissive dinuclear Cu(i) halide complexes with delayed fluorescence

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Graphical Abstract

Experimental and theoretical studies of the photophysical properties of three novel

emissive dinuclear Cu(I) halide complexes with thermally activated delayed

fluorescence (TADF) are reported.

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Experimental and theoretical studies of highly emissive dinuclear

Cu(I) halide complexes with delayed fluorescence

a,b

c

a, b

*a

Received 00th January 20xx,

Accepted 00th January 20xx

Liju Kang, Jin Chen, Teng Teng, Xu-Lin Chen, Rongmin Yu, Can-Zhong Lu

A series of luminescent homo-dinuclear Cu(I) halide complexes, [PPh

PPh PAr =(1-bis(2-methylphenyl)phosphino-2-diphenylphosphino)benzene) were synthesized from the reaction of

corresponding cuprous halide and the chelating bisphosphine ligand PPh PAr in CH CN. The complexes were structurally

2 2 2 2 2

PAr Cu(μ-X) CuPPh PAr ] (X= I (1), Br (2), Cl (3))

DOI: 10.1039/x0xx00000x

(

2

3

www.rsc.org/

characterized by X-ray single crystal analysis. Their photophysical properties were studied in detail. The Cu(I) atoms in

these complexes are four-coordinated and adopt a tetrahedral coordination geometry. In each complex, the copper

2 2 2

centers are bridged by two halide anions and each Cu(I) is chelated further terminally by a PPh PAr ligand. The[Cu(μ-X) Cu]

cores have similar butterfly-type configurations. The distances between the Cu(I) atoms in each complex are over 2.94 Å.

In the solid state, these complexes are highly emissive and exhibit bluish-green photoluminescence (emission peaks,

λmax=488nm(1), 482nm(2), 490nm(3)) with short lifetimes (4.9-5.9μs) and high quantum yields (ф = 0.42-0.95) at room

–

temperature. In this series complexes, the ligand-field strengths of the ions (I < Br <Cl ) do not have obvious effect on the

emission maxima. The studies of varied temperature emission spectra and decay behaviours of these complexes indicate

that the mechanism of their emissions involve two thermal-equilibrium excited states. At room temperature, the

complexes display thermally activated delayed fluorescences with short decay lifetimes. With a decrease of the

temperature, a significant increase of emission decay times by almost 2 orders of magnitude is observed. At temperatures

below T ≈ 100 K, the decay times of the studied complexes are over one hundred microseconds long, which indicates that

the emission originates mainly from a triplet state (T

complexes, an equilibriated 2 excited states model S

calculations suggest that the emission in the solid state originates from the (MLCT+XLCT+ILCT) excited states, in which

1

state). To interpret the varied temperature photophysics of these

0

←T ↔S →S was used. The results of the experimental and DFT

1

0

1

,3

1

3

emissive excited states, S and T, are in equilibrium with an energy difference of about 0.055 eV. The process of the

reverse intersystem crossing was estimated to be in the order of 2 ns

However, these materials are highly expensive and their metal

resources are very limited. As much cheaper alternative

Introduction

Phosphorescent complexes of noble metals, especially of materials, luminescent copper(I) complexes have attracted

iridium, have been investigated extensively particularly due much attention because of their interesting and excellent

to their uses as emissive dopants for light generation in emissive properties

，

1

2, 18-28

. Although the small spin-orbit

1

-10

organic light-emitting diodes (OLEDs) . With highly efficient coupling (SOC) effect of copper is not enough to promote

spin-orbit coupling, phosphorescent materials are superior efficient and short-lived phosphorescence, the energy gaps

over fluorescent materials because they can harvest both between the lowest singlet state and the lowest triplet state

2

9-31

singlet and triplet excitons in electroluminescent processes, ΔE(S

1

− T

1

) in many emissive Cu(I) complexes are small

, and

1

1-17

leading to a theoretically 100% internal quantum efficiency

.

the singlet state can be thermally activated at the expense of

the triplet state, leading to efficient and short-lived thermally

activated delayed fluorescence (TADF) at ambient

a.

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 35002, China. E-

mail: rongminyu@fjirsm.ac.cn; czlu@fjirsm.ac.cn.

Graduate University of Chinese Academy of Sciences, Beijing, 100049, China †

College of Life Science, University of Fujian Agriculture and Forestry, Fuzhou,

Fujian 350002, China

3

2-35

temperature

. With the capability to harvest both the

b.

c.

singlet excitons and the triplet excitons, the Cu(I) complexes

with TADF have been considered as promising emissive

Electronic Supplementary Information (ESI) available: Characterization data and

computation data. Crystallographic data have been deposited at the Cambridge

Crystallographic Data Center. CCDC reference numbers are 1057114, 1057120, and

36-38

dopants in electroluminescent devices

. Indeed, a few

1

057182 for complexes 1, 2, and 3, respectively. See DOI: 10.1039/x0xx00000x

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OLEDs with very impressive performance containing TADF Cu(I) core. However, the emissive properties of the complexes are very

3

9-46

complexes have been fabricated over the last few years

.

different from those found in dppb Cu(I) halide complexes. In this

In recent years, neutral Cu(I) complexes synthesized from paper, the structures, photophysical properties and theoretical

simple reactions of cuprous halides with various P- and N- calculations of the complexes are presented.

containing ligands have been investigated extensively because

Results and Discussion

of their interesting photophysical properties and good

2

2, 23, 29, 47-62

performances in OLEDs

. Among with, 1,2-

Syntheses and structures

bis(diphenylphosphino)benzene (dppb) and its derivatives

have been used in the syntheses of several interesting Cu(I)

The PPh

benzene with sodium NaPAr

NaPPh were synthesized from the reactions of respective

triarylphosphine and sodium in THF and used in situ. In the first step,

even though two equivalent amount of NaPAr was used, only one

2

PAr

2

ligand was prepared from the reaction of o-difluoro-

4

0

halide complexes. Ueno and co-workers reported halogen-

2

and NaPPh in two steps. NaPAr and

2

bridged binuclear complexes [Cu(μ-X)dppb]

These complexes exhibit intense

2

(X= I, Br, Cl).

bluish–green

2

6

3

photoluminescence in the solid state. Osawa and coworkers

2

modified the dppb by replacing the diphenylphosphino groups

with bis(2-alkylphenyl)phosphino groups, and obtained three

fluorine of o-difluoro-benzene was replaced by bis(2-

methylphenyl)phosphino group to form 1-fluoro-2-(bis(2-

methylphenyl)phosphino)benzene. In the second step, equal

new

methylphenyl)phosphino]benzene, 1,2-bis[bis(2-ethylphenyl)-

phosphino]benzene, and 1,2-bis[bis(2-

dppb

derivatives,

1,2-bis[bis(2-

equivalent amounts of

NaPPh

2

and 1-fluoro-2-(bis(2-

PAr

methylphenyl)phosphino)benzene were used, and the PPh

2

isopropylphenyl)phosphino]benzene]. By using these dppb

derivatives as chelating ligands, they obtained several highly

emissive three-coordinate copper(I) complexes that display

excellent photoluminescent performances in both degassed

product was obtained in moderate yield. Complexes 1-3 were

prepared from the reaction of equivalent amount of respective

cuprous halide and the PPh PAr ligand. After purification by

2

recrystallization, the complexes were obtained at moderate yields.

1

8, 64, 65

dichloromethane solutions and amorphous films

.

The crystal structures of the complexes were determined by X-

ray single crystal analysis. Selected bond distances and angles are

summarized in Table 1. Their ORTEP plots are shown in Fig.1. As

shown in Fig.1, the molecular structures of 1-3 are composed of

two Cu(I) atoms, two halide bridging ligands and two terminal bis-

phosphine ligands, respectively. The Cu(I) atoms are coordinated by

two halide anions and two P atoms from a terminal PPh PAr ligand,

Conventional OLEDs containing these complexes as dopants in

the emitting layer exhibited bright luminescence with very

high external quantum efficiencies (18.6-22.5%). The emission

maxima (λmax) of these dppb and modified dppb Cu(I)

complexes show the order I < Br < Cl, suggesting that λmax is

–

affected by the ligand-field strength (I < Br <Cl ). These

studies demonstrate that the introduction of alkyl substituent

at the ortho positions of dppb phenyl rings affects remarkably

the structures as well as the emissive properties of their

copper halide complexes.

2

resulting the formation of

a highly distorted tetrahedral

coordination with the angles of X–Cu–X and P–Cu–P largely

4

deviating from the usual tetrahedral value of 109.5° . The small P–

Cu–P angles of the complexes are only about 87.5° because of the

Here we report the synthesis of a new dppb derivative

2 2

small bite angle of the PPh PAr ligand. The dihedral angles

bisphosphine

ligand,

1-bis(2-methylphenyl)phosphino-2-

between the X–Cu–X plan and P–Cu–P plan are 88.37(2), 89.94(2)

and 89.74(2)° for 1, 2, and 3, respectively. The copper(I) atoms in

each complex are bridged by two halogen ligands to form a four-

diphenylphosphino)benzene, in which the two phosphine groups

are different. It is interesting to investigate the structures and

emissive properties of the Cu(I) halide complexes with this new

ligand. By using this unsymmetrical bisphosphine ligand, we have

obtained three new binuclear cuprous halide complexes. Similar to

membered butterfly Cu

are obviously different from a planar Cu

in this type of halogen-bridged copper complexes

2

X

2

ring The Cu

2

X

2

cores in these complexes

2 2

X geometry found usually

4

4, 66

. The torsion

binuclear complexes of [Cu(μ-X)dppb]

2

(X= I, Br, Cl), the present

angles of X–Cu–Cu–X that express the magnitude of the butterfly

studied complexes have structures with a butterfly [Cu(μ-X)

2

Cu]

2

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conformation deviating from the planar conformation are 154.83(3) different molecular structures (A and B as shown in Scheme 1). The

and 148.73(3)° for 1, 147.70(3)° for 2, and 148.55(4)° for 3. The terminal PPh PAr ligands are in a cis arrangement in A, while are in

2

Cu−X and Cu−P bond distances in these complexes are close to a trans arrangement in B. However, only the structures in cis

those observed in [Cu(μ-X)dppb] . The Cu ··· Cu distance is well- arrangement were found from the X-ray structural analysis for

2

known to exert an evident impact on the photophysical properties complexes 1-3 (as shown in Fig. 1).The molecular structures of

of the polynuclear cuprous halide complexes. The short Cu ··· Cu these complexes have ideally C

distance (less than 2.7 Å) usually generates the emissive CC (cluster- molecules of these compounds reside ideally in positions of C

centered) state. The Cu ··· Cu distances in complexes 1-3 are over only half-molecule independent in the asymmetric unit. The whole

.9 Å, implying the minor contribution of Cu ··· Cu interaction which molecules are generated through C symmetric operation. Complex

is usually observed for the complexes with a shorter Cu ··· Cu 1 crystallizes in the monoclinic group P2/n with Z = 4. There are two

2

symmetry. It is interesting that the

2

with

2

6

7

distance than the sum of van der Waals radii (about 2.8 Å) . A half-molecules in the asymmetric unit. Complexes 2 and 3

comparison of the interatomic distances of these complexes reveals crystallize in C2/C with asymmetric unit consisting of one half-

that the lengths of the copper halide bonds (Cu−X) decrease from 1 molecule.

(

2.611Å) to 2 (2.488 Å) to 3 (2.363 Å). This trend is also reflected in

Photophysical properties and DFT and TDDFT calculations for X-ray

structures

the Cu ··· Cu distances and the X ··· X distances, which is due to the

larger ionic radius of the heavier halide atom.

The photophysical data of the complexes are summarized in

Table 2, and the energy levels of them calculated from

electrochemical and photophysical data are listed in Table 3.

Absorption spectra and emission spectra of the complexes and free

Since the diarylphosphino substituent in the PPh

different, the combination of the butterfly Cu₂X₂core with two

terminal PPh PAr ligands were supposed to in the formation of two

2 2

PAr ligand are

2

Table 1 Selected bond lengths (Å) and angles (º) for 1-3.

a

complex

1

2

3

Cu1-X1

2.6114 (6)

2.270 (1)

2.327 (1)

3.175 (1)

4.0821 (4)

102.04 (2)

74.41 (2)

87.51 (3)

-154.83 (3)

2.6428 (5)

2.2761 (9)

2.3450 (9)

3.1034 (9)

4.1264 (4)

102.51 (2)

71.83 (2)

2.4896 (5)

2.2624 (1)

2.3134 (1)

3.0150(1)

3.7845(6)

99.43 (2)

74.85 (2)

87.56 (3)

-147.70 (3)

2.3667 (6)

2.3028 (6)

2.2606 (6)

2.9403 (6)

3.557 (1)

97.74 (2)

77.02 (2)

87.71 (2)

-148.55(4)

Cu1-P1

Cu1-P2

Cu1---Cu1

X1----X1

X1-Cu1-X1

Cu1-X1-Cu1

P1-Cu1-P2

87.22 (3)

X1-Cu1-Cu1-X1

-148.73 (3)

a

There are two half-molecules in the asymmetric unit of complex 1.

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transition from the lone pair orbital on phosphorous to an emptyπ

orbital on the o-phenlylene or phenyl rings of the ligand, and the

*

latter is the transition inside or between the o-phenlylene or phenyl

rings. The absorption spectra of complexes 1-3 are very similar to

each other and are more complicated and broader than that of the

free PPh PAr ligand. All complexes have an intense broad

2

absorption band centered at about 300 nm in the 250–310 nm

regions and a less intense shoulder at∼370 nm. The bands at about

3

00 nm are similar to the intense band observed in the spectrum of

the PPh PAr ligand, suggesting that these bands involve an intra-

2

ligand (IL) character. The absorption bands from 350 to 400 nm

which are not observed for the free ligand, are assigned to

transitions affected by the Cu(I) atoms and halide atoms. TDDFT

calculations indicate that the origin of the absorption bands

involves σ→ π * with metal-to-ligand charge transfer (MLCT)

character and p(X)→π* with halogen-to-ligand charge transfer

Fig. 1 ORTEP plots of complexes 1-3, all the H atoms are omitted for

clarity.

(

XLCT) character. Complexes 1-3 in degassed CH

2 2

Cl solutions at

room temperature give very weak green emission with the

λ

max

P

X

P

Cu

(A)

values are 541(1), 551(2), and 559 (3) nm. Because of the large

Stoke shifts of emission peaks (about 170 nm), low quantum yields,

and broad unstructured shapes, it was assumed that the

luminescent excited states of complexes 1-3 possess CT character,

and undergo a structural change from tetrahedral-like ground state

P

CuX

X

P

X

(B)

Cu

P

1

0

9

68-70

(

d ) to flattened excited state (d ) upon excitation

. It has been

shown that for Cu(I) emissive complexes, a prompt fluorescence

should not be observed because of a fast thermal equilibration

Scheme 1 The formation of binuclear Cu(I) halide complexes from

the combination of PPh PAr with CuX.

2

between the T

being in the order of 10 ps . This is also true for these studied

solutions are shown in Fig. 2 and Fig. S1. complexes as indicated by the observation that the emissions of the

The emission spectra of 1-3 at 77 and 298 K in the solid state are complexes are totally quenched in oxygen saturated CH Cl

1

and S states resulting from the processes of ISC

1

7

1

2 2

ligand in degassed CH Cl

2

shown in Fig. 3.

solutions.

The absorption spectrum of the free PPh PAr ligand exhibits a

Complexes 1-3 exhibit bluish green luminescence in the solid

2

4

-1

broad, intense band at 289 nm (ε = 10

M

cm ), which is state at 77 K and 298 K. The spectra are broad and unstructured,

characteristic of arylphosphine compounds. This band is assigned to suggesting that the emissive excited states have the charge-transfer

a mixed transition of n→π* and π→π*; the former is the

character. The emission quantum yields at room temperature in the

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Ple

D

as

a

e

lt

d

o

n

T

o

r

t

a

n

d

s

ju

a

s

c

t

m

io

a

n

r

s

gins

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solid state of complexes 2 and 3 are moderate, whereas compound reveals that the complexes contain different emissive processes

exhibits a very high value of ФPL = 95%. The emission maxima lie that are inter-exchangeable and thermally activated. It is obvious

at 488, 482, and 490 nm for complexes 1-3 at ambient temperature that the emissions are phosphorescence (T →S ) dominantly. In

1

0

respectively. The emission energy and shapes of the spectra for addition, the spectral blue shift of the emission toward higher

complexes 1-3 are similar, the same applies to the emission spectra temperatures indicates that a higher electronic state is thermally

at 77K. The emission maxima lie at 508, 503, and 506 nm for populated. The transition corresponding to the higher energy state

complexes 1-3 at 77K. An about 20 nm red shift of the emission carries a significantly large allowed-ness than the low-temperature

occurs for each of the complexes as temperature decrease from phosphorescence as shown by the decrease of the decay time upon

2

3, 47, 56, 72

2

98 to 77 K. According to the reported results

, the temperature increase. The drastic decrease of the emission

emission energy is affected by the nature of halogen ions directly lifetimes along with the blue shift of the emission maxima with

bound to copper atoms. With an increase in the ligand field increasing temperature indicate that all studied complexes at room

strength, the energy separation of the d-orbitals increases, leading

to a smaller MLCT energy and to red shift of the emission maxima.

Therefore, one would expect that the emission maxima in the series

of from I, Br, and Cl should be in the order I < Br < Cl. For examples,

the

λmax values of dinuclear complexes of [Cu(μ-X)dppb]

2

and

4

0, 66

[

Cu(μ-X)PNMe

2

]

2

(X = I, Br, Cl) are in the order of I < Br < Cl

.

However, in the present studied complexes, it is surprising that the

ligand field strengths of the halide bridging ions do not affect

obviously the emission energies of the complexes. DFT calculations

show that the stabilization of the HOMO in the series from Cl, Br,

and I does not differ in these studied complexes; and the HOMO

and the LUMO of these complexes are essentially at the same

energy, respectively. The results of the calculations are consistent degassed CH

with the experimental observation.

Fig.2 Absorption spectra of cuprous complexes and free ligand in

-5

Cl (c = 1×10 ) at room temperature.

2

The emission decay times measured at 298 K for the complexes

in solid states do not differ significantly and are very close to the

2

values found in the [Cu(μ-X)dppb] complexes. The lifetimes are

determined to be 4.9, 6.4 and 6.3 μs for complexes 1-3, respectively.

The microsecond magnitude of the lifetimes reveals that the

photoluminescence are related to triplet excited states. However,

at 77 K, the emission lifetimes of the complexes are very different.

The lifetimes are determined at 77 K to be 114.2, 188.7, and 385.8

μs for complexes 1-3, respectively. The differences are related to

the spin-orbital coupling being differently effective in the

complexes. It is interesting that the order of decay times 1< 2 < 3 is

consistent with that one would expect from the decrease of the

spin-orbital coupling effects in the order of I > Br > Cl. With a

temperature decrease from 298 to 77K, the emission decay times

Fig. 3 Emission spectra of cuprous compounds in powder states at

7K and 298K (λ_e_x_c= 330nm).

increase drastically from

a few microseconds to hundreds

7

microseconds. A comparison of the decay times at 77 and 298 K

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Table 2 Photophysical data of complexes 1-3 at 298 K.

Solution (in degassed CH Cl )

Crystalline powder

τ/μs

4.9

2

-3

-1

complex

λ_a_b_s/nm (ε×10 M cm )

307(18), 373(5.6)

309(17), 373(3.3)

309(22), 375(4.0)

λ_e_m

488

Ф(%)

95

1

2

3

541

551

559

482

490

6.4

6.3

50

42

Table 3 Energy levels and gaps of complexes 1-3.

a/

b

c/

d

1

e

complex

ΔE_geV

HOMO /eV

LUMO eV

S /eV

T /eV

ΔE /eV

st

1

2

3

2.73

2.85

2.71

-4.81

-4.75

-4.82

-2.08

-1.90

-2.11

2.83

2.84

2.78

2.79

2.73

0.05

0.04

2.77

a

b

Estimated from the onset wavelengths of the absorption spectra measured in CH Cl . Calculated from the oxidation potentials of

2

c

d

cyclic voltammetry. Calculated from the HOMO energy levels and ΔE . Estimated from the onset wavelength of the 298K emission

g

e

spectra measured as powders. Estimated from the onset wavelengths of the 77K emission spectra measured as powders.

temperature show thermally activated delayed fluorescence

The calculation reveals that the bands of the lower-lying

5

(TADF), which has been observed often from Cu(I) complexes 20 electronic transitions of the complex mainly involve the

and organic donor-acceptor compounds with small S -T energy

transitions from HOMO and HOMO-1 to unoccupied orbitals

LUMO and LUMO+1. Orbital component analysis shows that

both the HOMO and HOMO-1 have main contributions from Cu

atoms, X atoms, and P atoms of PPh PAr , while the LUMO and

1

3

, 27, 29

gaps

The electronic structure, and the type and character of

electronic transition and the composition of the involved orbitals

can be elucidated by carrying out DFT and TDDFT calculations.

Because structural changes are minimal upon excitation in the

crystalline solid state, no structural optimization was performed

for DFT and TDDFT calculations in this study. Calculated energy

levels, oscillator strengths (f), and orbital transition analyses for

selected lower-lying transitions for complexes 1-3 are listed in

Table 4 and Table S3. From the calculations, HOMO, HOMO-1,

LUMO, and LUMO+1 in the structure determined by X-ray

analysis are shown in Fig. 4 and Fig. S2.

2

3

5

0

LUMO+1 are essentially localized at the PPh PAr ligand. The

2 2

1

0

5

distinct distribution of the frontier orbitals indicates that the

transition processes involving these orbitals possess significant

CT character. Natural transition analyses were performed to

investigate the origins of the transitions for the complexes.

According to the computational results, the lower lying

transitions for these complexes all contain substantial MLCT and

XLCT characters. Maps of the hole and electron in S and T₁

1

states of the crystal structure of 1 are illustrated in Fig. 5.

The maps of S and T are similar to each other. The hole in 1

1

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is mainly confined to the orbitals of the σ bonds composed of

the copper atoms, the iodine atoms and the phosphorus atoms;

and a little is distributed over the phenyl groups of the PPh PAr

slightly in fluorescent and phosphorescent processes,

respectively. In contrast, the fraction contributions from XLCT

vary more evidently. The fraction contributions from XLCT in

2

ligand. The electron distribution lies mainly on the o-phenylene 25 S →S process are 23, 17, and 14%, and in T →S process are 19,

1

0

1

0

5

and the phosphorus atom. The contributions from σ→π* and

π→π* transitions to fluorescence (S →S ) were calculated to

15, and 13%, for complexes 1, 2, and 3, respectively. The order

of XLCT contributions to the photoluminescent processes, 1 > 2 >

1

0

3

^,ⁱ^s^t^h^e^s^a^m^e^a^s^t^h^e^o^r^d^e^r^o^f^t^h^e^ФPL values at room

be 70% and 18%, respectively. The σ→π* transition in S →S

1

0

temperature. This suggests that the XLCT should play vital role in

0 the emissions of these studied complexes.

contains contributions from MLCT (27%), XLCT (24%) and

3

ILCT(17%). The contributions from σ→ π * and π → π *

1

0

5

transitions to phosphorescence (T →S ) were calculated to be 66%

1 0

and 21%, respectively. The σ→π* transition in T →S consists of

1

0

contributions from MLCT (26%), XLCT (19%) and ILCT(19%).

Therefore, the lowest excited states of complex 1 are

1,3

appropriate to be assigned as (MLCT+XLCT+ILCT) states. It is

noteworthy that the contribution from σ→π* transition in

T →S decreases from 70% to 66% as compared with that in

1

0

S →S . NTO maps and origins of photoluminescence for

1

0

complexes 2 and 3 are similar to those of 1 (Fig. S3 and S4). The

contributions from the σ→π* transition in S →S and T →S

1

0

1

0

Fig. 4 HOMO, HOMO-1, LUMO, and LUMO+1 of complex 1 in the

2

3

0

5

are 69 and 67% for 2, and 70% and 57% for 3, respectively. The

structure determined by X-ray analysis.

fraction contributions from MLCT in these complexes change

Table 4 Calculated energy levels, oscillator strengths (f), and orbital transition analyses for selected lower-lying transitions for complex

1

.

states

S1

λ_c_a_l(nm)

370.63

f

assignments

MLCT

27.0%

XLCT

1

0.0204

HOMO→LUMO (82%)

23.5%

HOMO-1→LUMO+1 (13%)

HOMO→LUMO +1(82%)

HOMO-1→LUMO (12%)

HOMO→LUMO +2( 88%)

HOMO→LUMO+3 (76%)

HOMO-1→LUMO+2 (13%)

HOMO-1→LUMO+1 (82%)

HOMO→LUMO (12%)

S2

370.19

0.0085

24.6%

23.6%

S3

S4

361.28

358.03

0.0417

0.0085

24.6%

23.0%

23.5%

23.4%

S5

T1

T2

349.88

384.40

383.75

0.0004

36.2%

26.3%

25.3%

23.1%

20.1%

20.2%

0

HOMO-1→LUMO+1 (18%)

HOMO→LUMO (62%)

HOMO-1→LUMO (21%)

HOMO→LUMO+1 (65%)

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ARTICLE

of the emission lifetimes of these complexes are shown intuitively

from the Figure. At sufficient low temperature, the emissive state is

1

dominated by the lowest triplet state (T ). The corresponding

transition has long decay time because the transition from the

lowest triplet state to the singlet ground state is spin-forbidden. As

mentioned above, the prompt S

due to the fast S →T intersystem crossing. However, upon

temperature increase, the thermal activation of the short-lived

singlet state S that decay irradiatively to give the TADF, is

evidenced. Even though the emissive process involves the transition

of S →S , it is not the normal prompt S →S fluorescent process.

Because of the simultaneous existence of the fast ISC of S →T and

1 0

→S fluorescence does not occur

1

0

1

0

1

Fig.5 Natural transition orbital pairs for (A) the lowest triplet excited

state and (B) the lowest singlet excited states of 1 in the structure

determined by X-ray analysis. The generation probabilities are 93

and 98%, respectively.

the thermally activation of T

happen between S and T . As a result, the emitting processes can

be described as S ←T ↔S →S . With the use of the equilibrium

constant (K), the observed emissive lifetime τ_o_b_sis expressed as

1 1

→S , a thermal equilibrium will

1

0

1

0

4

0

（

ꢇ

ꢈ

ꢉꢇ

ꢊ

ꢋꢌ

^ꢀꢁꢂꢃ^ꢄ^ꢅ

ꢆ

(1)

As mentioned above, NTO analyses show that the lowest excited

states, S and T , have substantial charge transfer character. The

ꢅꢉꢋ

1

corresponding transitions should induce distinct spatial separations

of the involved orbitals, leading to relatively small exchange

1 1

integrals and consequently to small energy gaps ΔE(S − T )

between the T

1

and the S

1

. The TDDFT calculations reveal that the

and T states are very small (0.12, 0.15,

energy gaps between the S

1

and 0.17 eV for complexes 1, 2, and 3, respectively). This result

confirms further the major CT character of the emissive processes

1 1

from S and T excited states, and indicates theoretically that these

materials can show a very efficient thermally activated delayed

fluorescence.

Fig. 6 (a). Temperature-dependent emission lifetimes of complexes

-3 powders. The solid line is a fit curve according to eq 1. (b)

Energy levels for ground-state and lowest excited state of

1

Temperature dependence of emissive decay times and TADF

parameters fitting

complexes 1-3. Herein,

τ ,

PF

DF,

τ τP, ISC, RISC and ΔEst represents rapid

Further understanding of the emission properties of these fluorescence decay time, delayed fluorescence decay time,

complexes is obtained from the study of their emissive decay times phosphorescence decay time, intersystem crossing, reverse

as a function of temperature in the range between 77 and 298K. intersystem crossing and the energy gap between S1 and T1 state.

The results are depicted in Figure 6a. The temperature dependence

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understanding of the emission process. For the studied Cu(I)

complexes, the

τ

(T) fitted values are close to the measured value at

at this

7

7K, which confirms that the dominated excited state is T

1

temperature. The fit values of ΔEST are in good agreement with the

values determined from the emission spectra. The small energy gap

between S₁and T₁state facilitates up-conversion of T →S and

1

therefore leads to pronounced TADF at ambient temperature. For

the decay time (fit data) of the singlet state τ , values of 17 to 58 ns

S

Fig. 7 The temperature-dependent emission intensity occupancies

of the singlet state S (delayed fluorescence) and the triplet state T

phosphorescence) related to the total intensity for complex 1. The

are determined. The values are 3 orders of magnitude smaller than

1

the fit values of τ . These results support the presented model in

T

(

which the higher- and lower- energy states are assigned to a spin-

1 1 1

ratio of S is calculated using eq. 5, and that of T using I(T )/(I(total)

allowed S

the short times of τ

Table 5 Parameters for the temperature dependent emission of the transition in metal-containing fluorescent complexes, indicating

1 1

and a spin-forbidden T state, respectively. Moreover,

=

1

1- I(S )/(I(total)

S

1 0

are in the lifetime range of normal S →S

complexes 1-3 in the solid state.

that the step of S

emissive process of T

are in a thermal equilibrium. At room temperature, the decay

times of the delayed fluorescence range from 4.9 to 6.5 μs. These

values are 2 orders of magnitude larger than the fit τ values and

1

→S

0

transition is the rate determining step in the

1

↔S →S in which the excited states of T and

1

0

1

2

3

S

1

a

ΔEST /eV

0.050

0.040

ΔEST/eV

0.056

114.20

114.94

0.017

0.055

188.70

193.54

0.036

0.056

385.80

405.85

0.058

b

τ(77K) /μs

τ_T/μs

S

governed by the singlet decay times, the triplet decay times and the

equilibrium constant at the temperature as shown in eq. 4. The

equilibrium constant at 298K for complex 3 is calculated by eq. K =

τ

S

/μs

A

0.019

0.027

0.038

a

ΔEst : Energy determined from emission spectra. b. Decay

time measured at 77K.

Aexp(-ΔE_S_T/

κ

B

T) to be 0.00223. Then, the population ratio in S1

where k and k denote the radiative rate constants at T and

T

S

state at 298 K is obtained to be only 0.223% by eq. K = [S

1

]/[T1].

B

S states, respectively. Since K = Aexp(-ΔE_S_T/κ T), eq 1 is

Since K = k_R_I_S_C/k_I_S_Cand the process of ICS is in the order of 10 ps, the

process of the RISC is estimated to be in the order of 2 ns. The

extremely fast processes of ICS and RISC lead to the instantaneously

readily transformed to

ꢓ∆ꢔꢊꢈ

ꢍꢇ

ꢈ

ꢉꢇ

ꢊ

ꢎꢏꢐꢑꢒꢍ

ꢓ∆ꢔ

ꢌ

ꢕꢖꢗꢈ

ꢀ_ꢁ_ꢂ_ꢃ^ꢄ^ꢅ

ꢆ

(2)

ꢊꢈ

ꢅꢉꢏꢐꢑꢒꢍ

ꢌ

ꢕꢖꢗꢈ

equilibration between S

^q^uⁱ^c^k^e^r^t^h^aⁿ^t^h^a^t^o^f^T1 combined with the instantaneously

equilibration between S and T , results that the emission of the

complex occurs mainly from the singlet state although the

population ratio of S at 298K is very small. Using fitted parameters,

1 1 1

and T . The decay of S being much

where A is a constant, κ

absolute temperature, and ΔEST is the energy gap between T

B

is Boltzmann constant, T is the

and S states. Since k

T

=1/τ

T

, k

S

=1/τ

S

, equation (2) can be

1

written as

ꢘ

ꢓ∆ꢔꢊꢈ

ꢕꢖꢗꢈ

ꢉ

ꢎꢏꢐꢑꢒꢍ

ꢌ

ꢙꢈ ꢙꢊ

ꢀ_ꢁ_ꢂ_ꢃ^ꢄ^ꢅ

ꢆ

ꢗꢗ(3)

ꢓ∆ꢔ

ꢊꢈ

ꢅꢉꢏꢐꢑꢒꢍ

ꢌ

1

ꢕꢖꢗꢈ

the singlet component occupancies of luminescence (I(S )/I(total)

1

or

54

can be estimated by

ꢓ∆ꢔꢊꢈ

ꢕꢖꢗꢈ

ꢅꢉꢏꢐꢑꢒꢍ

ꢘ

ꢙꢈ ꢙꢊ

ꢌ

ꢅ

ꢚꢍS ꢌ⁄ꢚꢍtotalꢌ ꢆ

ꢙ

^ꢆꢍꢅꢉ

ꢙ

ꢊ

(5)

^ꢀ^ꢁꢂꢃ

ꢆ _ꢘ

ꢓ∆ꢔ

(4)

ꢅ

ꢊ

ꢊꢈ

ꢍꢅꢉ_ꢙ

ꢌ

ꢉ

ꢎꢏꢐꢑꢒꢍ

ꢌ

ꢎꢛ

ꢓ∆ꢔꢠꢡ

ꢕꢖꢗꢈ

ꢈ

ꢙꢈꢎꢜꢝꢞꢟꢍ ꢈ ꢌ

ꢢ

ꢖ

Fig. 6 (a) shows the plots of τ versus T for complexes 1-3. With

the use of a weighted linear least-square fitting method, the complex 1, the plots shown in Fig. 7 are obtained. Similar plots for

parameters, τ , τ , A, and ΔEST in eq 4 are determined from the plots complexes 2 and 3 are also obtained and shown in Fig. S5. At low

Table 5). An analysis of these data allows us to have a better temperature (< 50 K), only T₁phosphorescence occurs. With a

Applying eq. 5 and using the fitted parameters as determined for

T

S

(

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temperature increase, the intensity contribution of T decreases, Analysing the varied temperature decay times with a S ←T₁↔S₁→S₀

1

0

while the intensity of S

1

state increases. The singlet components at model in which equilibrium between T

1

and S

1

was assumed gave

2

98 K are calculated to be 93%, 94% and 97% for complexes 1, 2, insight into the mechanism of the emissions. From the model and

and 3, respectively, indicating that almost all intensity at 298 K parameters fitting using a weighted linear least-square fitting

originates from the singlet states. method, the fit values of τ , τ , A, and ΔE were obtained. The fit

T

S

ST

values of τ

supporting the reasonability of the model. The short fit times of τ

are in the lifetime range of normal S →S transition, indicating that

the step of S →S transition is the rate determining step in the

emissive processes of T →S in which the excited states of T of

are in a thermal equilibrium. From the fit values, the equilibrium

constants of T are calculated, and the rate constants of ISC and

RISC are estimated.

T

and ΔEST agreed well with the experimental results,

2

1, 27, 73

In previous reports

, equation

S

ꢘ

∆ꢣꢠꢡ

ꢤꢖꢈ

1

0

ꢅꢉ

ꢥ

ꢐꢑꢒ ꢗꢍ ꢄ

ꢌ

^τ^ꢁ^ꢂ^ꢃ^ꢆ^ꢘꢉ

ꢐꢑꢒꢗꢍ

ꢓ∆ꢣ ꢗꢗꢗ (6)

ꢘ

ꢠꢡ

ꢌ

1

0

ꢙꢦ ꢥꢙꢧ

ꢤꢖꢡ

1

↔S

1

0

1

was used to fit the experimental data with the assumption that

S

1

there is a fast thermalization between the S₁and T , and the

1

↔S

1

contribution ratio of S

substates in T . However, the fit emissive decay time of S

overestimated because the population in T and S are assumed to

1 1

/T is 1/3 by considering that there are 3

1

is often

The thermally delayed fluorescence materials are not only

intriguing in the fundament aspects, but also useful in practical

1

be totally controlled by thermal statistics, and the intersystem

crossing (ISC) and the reverse intersystem crossing (RISC) are

neglected. As compared, the fitting by using eq. 4 is more

reasonable.

1 1

applications. The instant equilibrium between the S and T states

empowers these materials to use fully all of the excitons in

electroluminescent processes. The TADF Cu(I) complexes

possessing high emitting quantum yields and relatively short decay

times are promising emitters in OLEDs. The solid and deep

understanding of the emitting process provided from practical and

theoretical studies are important in guarding the further studies

and to fully make use of the complexes in various areas.

Conclusions

In this article, a series of Cu(I) halide complexes 1-3 with a new

dppb derivative bisphosphine ligand were studied in deep. X-ray

structure characterization shows that the structures of these

complexes are similar to the binuclear dppb cuprous halide Experimental Section

4

0

complexes reported previously.

Detailed studies on the

Materials and methods

photophysical properties of these complexes show that the

introduction of methyl substituents into the phenyl rings of one of

the biphenyl phosphine groups of dppb has evident influence on

emissive properties. A 14, 38, and 43 nm blue shift of the emission

occurs for complexes 1, 2, and 3, respectively, as compared to their

respective dppb analogues.

Copper(I) iodide, copper(I) bromide, copper(I) chloride,

triphenylphosphine,

o-difluoro-benzene,

tri(2-

methylphenyl)phosphine, 1-methylnaphthalene, and all solvents

are commercially available and were used without further

1

31

treatment. H and P NMR spectra were recorded on a Bruker

Avance III 400MHz NMR spectrometer. Elemental analyses (C, H, N)

were carried out with an Elemental Vario EL III elemental analyze.

Theoretical calculations reveal that the lowest transitions of both

singlet and triplet states have substantial charge transfer

contributions including MLCT, XLCT, and ILCT. Distinct spatial

-

1

The infra-red spectra in the region of 4000-400 cm were obtained

in KBr discs with a Perkin-Elmer FT-IR Spectrum One. Cyclic

voltammetry was carried out in a gastight single-compartment

three-electrode cell with a BAS Epsilon Electrochemical Analyzer at

room temperature. A glassy carbon disk and a platinum wire were

separations of the involved orbitals in T

result in relatively small exchange integrals between the orbitals,

and consequently to small energy gaps E(S ) between the T

1 0 1 0

→S and S →S transitions,

Δ

1

−

T

1

and the S . Results of experimental and theoretical calculations

used as working and auxiliary electrodes, respectively. And the

indicate that the emissions of these complexes originate from two

thermally balanced singlet and triplet states and display TADF.

+

reference electrode was Ag/Ag (0.1M of AgNO

3

in acetonitrile). The

1

0 | J. Name., 2012, 00, 1-3

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CV measurements were performed in anhydrous and nitrogen- extracted with dichloromethane and water. The combined organic

saturated dichloromethane solutions with 0.05M n- extracts were dried with MgSO , filtered and evaporated to give the

4

tetrabutylammonium perchlorate (TBAP) and 0.05mM cuprous crude product. The product was purified by chromatography on

complexes. Photoluminescence spectra were recorded on a HORIBA silica gel using petroleum ether and dichloromethane (5:1) to give a

1

Jobin-Yvon FluoroMax-4 SPECTROMETER and an Edinburgh white product, (1.83 g, 47%). H NMR (400MHz, CDCl ): δ 2.28 (s,

3

1

Analytical instrument FLS920, respectively. The UV-vis absorption 12H), 6.75-6.72 (m, 2H), 7.23-6.98 (m,20H). P NMR (CDCl

spectra were recorded with a Perkin- Elmer Lambda 45 UV/vis δ -13.27 (d, J=159.95Hz), -27.94 (d, J=159.95Hz).

spectrophotometer. The lifetimes of powder samples at different

3

, 298K):

Synthesis of cuprous complexes

temperatures (77K-298K) were carried out by an Edinburgh

Analytical instrument FLS920 with a Picosecond Laser Diode. The PL

All the three cuprous complexes were synthesized according to the

following general procedure: A mixture of CuX (X=I, Br, Cl) (1 mmol)

quantum yields, which were defined as the number of photons

emitted per photon absorbed by the system were measured by

FluoroMax-4-equipped with an integrating sphere.

2 2 3

and PPh PAr (1 mmol) in 20 ml of CH CN was stirred at room

temperature for 30min, and the solvent was then evaporated under

vacuum. The crystals of these complexes were obtained by diffusion

Synthesis

of

1-bis(2-methylphenyl)phosphino-2-

PAr

of CH CN into CH2Cl2 solution of the products.

3

diphenylphosphino)benzene (PPh

2

)

[

PPh

2

PAr

2

Cu(μ-I)

2

CuPPh

2

PAr

2

] (1). Pale yellow crystals. Yield:

): δ 2.56 (s, 12H), 7.58-7.21

): δ 56.81 (s), 47.38 (s). Anal.

Calcd C H Cu I P : C, 57.79; H, 4.24. Found: C, 57.77; H, 4.38. IR

The PPh

In the first step, a mixture of tri(2-methylphenyl))phosphine (10.0 g,

2.8 mmol), 1-methylnaphthalene (0.25 g, 1.77 mmol) and fresh-cut

sodium pieces (3.50 g, 150 mmol) in anhydrous tetrahydrofuran

THF) (100 ml) in a 250 ml round-bottom flask was stirred under

2 2

PAr compound was synthesized from two successive steps.

1

2

6.0 mg, 48%. H NMR (400MHz, CDCl

3

31

(

m, 44H). P NMR (400MHz, CDCl

3

6

4

56

2 2 4

-

1

(

cm ): 3420w, 3046m, 1434s, 1092m, 748s, 515s

(

7

4

reflux under argon atmosphere over 24 h . The excess sodium was

[

PPh

2

PAr

2

2 2 2

Cu(μ-Br) CuPPh PAr ] (2). Pale yellow crystals. Yield:

removed by transferring the solution to another dried round-

1

2

8.6 mg, 50%. H NMR (400MHz, CDCl

3

): δ 2.55 (s, 12H), 7.58-7.21

bottom flask by using a stainless steel cannula. The resulting NaPAr

2

31

(

m, 44H). P NMR (400MHz, CDCl ): δ 57.64 (s), 47.97 (s). Anal.

3

in THF as a dark-red solution was used without further purification.

To the above dark red solution, o-difluoro-benzene (1.87 g, 16.4

mmol) was added. The mixture was refluxed for three hours, and

then stirred overnight at room temperature. After completion of

the reaction, a few drops of MeOH were added into the solvent.

2 2 4

Calcd for C64H56Cu Br P : C, 62.19; H, 4.57. Found: C, 61.82; H, 4.68.

-1

IR (cm ): 3745w, 3047m, 2362w, 1434s, 1094m, 743s, 511s.

2 2 2 2 2

[PPh PAr Cu(μ-Cl) CuPPh PAr ] (3). Pale yellow crystals. Yield:

2

6.9 mg, 51%. 1H NMR (400MHz, CDCl ): δ 2.55 (s, 12H), 7.58-7.21

3

1

(

m, 44H). P NMR (400MHz, CDCl

3

): δ -13.10 (d，283.2Hz), -25.26

2 2

The reaction mixture was extracted with CH Cl and water. The

d, 283.2Hz). Anal. Calcd for C H Cu Cl P : C, 67.01; H, 4.92. Found:

combined organic extracts were dried over MgSO After removal of

64 56

2

2 4

4

.

-1

C, 66.51; H, 5.03. IR (cm ): 3745w, 3047m, 1435s, 1272w, 1094m,

the solvent, the crude was purified by column chromatography on

silica gel to give the product as a white powder (3.0 g, 9.7 mmol). In

7

43s, 511s.

2

the second step, to a cool NaPPh solution prepared from the

X-ray structure determination

reaction of triphenylphosphine (2.62 g, 5.0 mmol) and sodium (1.40

g, 60 mmol) (excess Na was removed after the 12 h reaction) in

refluxed anhydrous THF (30 mL) were added the white product

from the first step. The mixture was stirred under reflux for 2 h and

at room temperature overnight. After completion of the reaction, a

few drops of MeOH were added into the solution. The solvent was

removed by vacuum evaporation, and the resulting solid was

Diffraction data were collected on a SuperNova, Dual, Cu at zero,

Atlas diffractometer with Cu Kα radiation (λ=1.54184Å). The

structures were solved by direct methods with SHELXS-97 and

refined by full-matrix least-squares methods with SHELXL-97

75

program package . All the non- hydrogen atoms were located with

successive difference Fourier synthesis and refined anisotropically.

J. Name., 2013, 00, 1-3 | 11

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Hydrogen atoms were added in idealized positions. Selected 12. F. Wu, J. Li, H. Tong, Z. Li, C. Adachi, A. Langlois, P. D. Harvey, L.

Liu, W.-Y. Wong, W.-K. Wong and X. Zhu, J.Mater Chem C, 2015,

crystallographic data are listed in Table S1.

3

, 138-146.

1

3. C. C. Hsu, C. C. Lin, P. T. Chou, C. H. Lai, C. W. Hsu, C. H. Lin and

Y. Chi, J. Am. Chem Soc, 2012, 134, 7715-7724.

Computational methodology

4. P.-T. Chou, Y. Chi, M.-W. Chung and C.-C. Lin, Coord. Chem. Rev

,

2

011, 255, 2653-2665.

5. J. Zhuang, W. Li, W. Wu, M. Song, W. Su, M. Zhou and Z. Cui,

New J.Chem., 2015, 39, 246-253.

6. D. Escudero and D. Jacquemin, Dalton Trans, 2015.DOI:

10.1039/C4DT03804E.

7. F. Nastasi, F. Puntoriero, M. Natali, M. Mba, M. Maggini, P.

Mussini, M. Panigati and S. Campagna, Photochem & Photobi

Sci, 2015.

The density functional theory (DFT) and time-dependent DFT (TD-

DFT) calculations were performed with the Gaussian 09 program

7

6

package , using the hybrid Becke three-parameter Lee-Yang-Parr

7

7, 78

(B3LYP) functional level

. The input structures were extracted

from the X-ray crystallographic data. In all calculations, the

relativistic effective core potential (RECP) and the associated basis

1

8. M. Osawa, I. Kawata, R. Ishii, S. Igawa, M. Hashimoto and M.

Hoshino, J.Mater Chem C, 2013, 1, 4375-4383.

9. F. Dumur, Org.Electron, 2015, 21, 27-39.

7

9

set Lanl08 (f) and Lanl08(d) , which are the revised version of 20. P. C. Ford, E. Cariati and J. Bourassa, Chemi. Rev., 1999, 99,

3

625-3648.

original Hay-Wadt basis set, were employed for the Cu(I), Br and I

atoms. All-electron basis set of 6-31G* was used for other non-

2

1. X.-L. Chen, C.-S. Lin, X.-Y. Wu, R. Yu, T. Teng, Q.-K. Zhang, Q.

Zhang, W.-B. Yang and C.-Z. Lu, J.Mater Chem C, 2015, 3, 1187-

1

195.

metal atoms of Cl, P, O, C and H. Visualization of the optimized 22. D. Volz, M. Nieger, J. Friedrichs, T. Baumann and S. Bräse,

Langmuir, 2013, 29, 3034-3044.

3. G. F. Manbeck, W. W. Brennessel, C. M. Evans and R. Eisenberg,

Inorg. Chem, 2010, 49, 2834-2843.

4. S. Igawa, M. Hashimoto, I. Kawata, M. Yashima, M. Hoshino and

M. Osawa, J. Mater. Chem. C, 2013, 1, 542-551.

5. C. Femoni, S. Muzzioli, A. Palazzi, S. Stagni, S. Zacchini, F. Monti,

G. Accorsi, M. Bolognesi, N. Armaroli, M. Massi, G. Valenti and

M. Marcaccio, Dalton Trans, 2013, 42, 997-1010.

structures and frontier molecular orbitals were performed by

2

GaussView. The partition orbital composition was analyzed by using

8

0

Multiwfn 2.4 program .

Acknowledgements

26. Q. Zhang, T. Komino, S. Huang, S. Matsunami, K. Goushi and C.

Adachi, Adv. Funct. Mater., 2012, 22, 2327-2336.

This project is supported by the Chinese Academy of Sciences

2

7. C. L. Linfoot, M. J. Leitl, P. Richardson, A. F. Rausch, O. Chepelin,

F. J. White, H. Yersin and N. Robertson, Inorg. Chem, 2014, 53,

10854-10861.

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