Theoretical investigation on the photophysical properties of N-heterocyclic carbene iridium (III) complexes (fpmb)xIr(bptz)3-x (x = 1−2)

Theoretical Investigation on the Photophysical Propertiesof N-Heterocyclic Carbene Iridium (III) Complexes(fpmb)xIr(bptz)3-x (x 5 122)

Qi Cao,[a] Jing Wang,[a] Zhao-Shuo Tian,*[a] Zai-Feng Xie,*[c] and Fu-Quan Bai*[b]

In the search for efficiently phosphorescent materials, this article

presents a rational design and theoretical comparative study of

some photophysical properties in the (fpmb)xIr(bptz)3-x (x ¼ 1–

2), which involve the usage of two 2-pyridyl triazolate (bptz)

chromophores and a strong-field ligand fpmb (fpmb ¼ 1-(4-

difluorobenzyl)-3-methylbenzimidazolium). The first principle

theoretical analysis under the framework of the time-dependent

density functional theory approach is implemented in this article

to investigate the electronic structures, absorption and

phosphorescence spectra. It is intriguing to note that 1 and 2

exhibit theirs blue phosphorescent emissions with maxima at 504

and 516 nm, respectively. Furthermore, to obtain the mechanism

of low phosphorescence yield in 1 and estimate the radiative rate

constant kr for 2, we approximately measure the radiative rate

constant kr, the spin-orbital coupling (SOC) value, DE (S � T), and

the square of the SOC matrix element (\WS1.HSO.WT1[2 ) for

1 and 2. Finally, we tentatively come to conclusion that the switch

of the cyclometalated ligand from the main to ancillary chelate

seems to lower the splitting DE (S � T) in the current system.

VC 2012 Wiley Periodicals, Inc.

DOI: 10.1002/jcc.22935

Introduction

Heavy transition metal compounds recently have been paid a

great deal of attentions on a variety of photonic applications. In

particular, cyclometalated Iridium (III) organometallic complexes

have been widely investigated for the excellent candidates for

light-emitting electrochemical cells,[1–3] organometallic light-emit-

ting diodes (OLEDs),[1–4] luminescent sensors, and so forth. Similar

to other heavy transition metals, such as ruthenium, osmium,

and platinum, Iridium (III) is capable to harvest both singlet and

triplet states from electrically generated excitons, thus giving rise

to an internal quantum efficiency of nearly 100%, whereas fluo-

rescent molecules can only utilized the singlet excitons and can-

not exceed a maximum quantum yield of 25% (according to spin

statistics).[5–7] In addition, tuning of emission colors over the

entire visible spectra, it has been well documented that, has

been easily achieved by ingeniously modifications of the cyclo-

metalated and/or ancillary ligands. As a result, there is a continu-

ous trend of shifting research endeavors focusing on these

cyclometalated iridium complexes.

Since the manufacture of a full color display requires red-

green-blue colors, a great number of elegant studies have

focused on the effect of ancillary chelates, with an aim to

achieve high efficiency phosphors. Therefore, many green- and

red-emitting phosphors have been synthesized during the

past decade. However, among all three primary colors, blue-

emitting phosphors have long remained a formidable challenge,

and only a few reports on room-temperature blue emitters

were reported.[8–11] Accordingly, some key criteria should be

realized with an aim to achieve highly efficient blue phosphores-

cence.[12] Of prime importance is to enhance the metal-to-ligand

charge transfer (3MLCT) transition energy in the triplet mani-

fold.[12–14] The participation of the d metal orbital in the

transition promotes the coupling of the orbital angular momenta

to the electron spin, so that the possibility of the T1 ! S0 singlet

electron transition is achieved through the large intersystem

crossing, leading to a sharp decrease in the radiative lifetime and

hence enhancing the possibility of high quantum yield.

Recently, the most investigated examples, bis(40, 60-difluoro-phenylpyridinato) iridium (III) picolinate (FIrpic)[15–17] and

bis(40, 60-difluorophenylpyridinato)iridium (III) tetra(1-pyrazolyl)-

borate (FIr6),[18,19] have become the excellent materials for

greenish-blue and sky-blue phosphorescent OLEDs. Further

endeavors were undertaken to substitutigetusernamng the

picolinate ligand with other ancillary chelates such as triazolate

or tetrazolate ions to afford heterocyclic complexes FIrtaz3,

[19,20] and FIrN4.[20] In the search for the promising phosphors,

in fact, we have long observed that the cyclometalated 2-pyri-

dyl triazolate chelate possesses a much large intraligand p–p*energy gap.[21] As compared with the maximum emission

wavelength of well-known FIrpic, these ingenious designs

[a] Q. Cao, J. Wang, Z.-S. Tian

Information Optoelectronics Research Institute, Harbin Institute of

Technology at Weihai, Weihai 264209, People’s Republic of China

E-mail: [email protected]

[b] F.-Q. Bai

State Key Laboratory of Theoretical and Computational Chemistry, Institute

of Theoretical Chemistry, Jilin University, Changchun 130023, China


[c] Z.-F. Xie

AU Optronics Trade (Shanghai) Corporation, Suzhou 215021, China


Contract/grant sponsor: The Fundamental Research Funds for the

Central Universities; Contract/grant number: HIT. BRET. 2010014;

Contract/grant sponsor: National Natural Science Foundation of China;

Contract/grant number: 21003057

VC 2012 Wiley Periodicals, Inc.

1038 Journal of Computational Chemistry 2012, 33, 1038–1046 WWW.CHEMISTRYVIEWS.COM

FULL PAPER WWW.C-CHEM.ORG

have produced a blue shift of � 10 nm. To our surprise, the

lowered quantum yield was noticed in some cases, which pro-

vides a limit to the fabrication of true-blue phosphorescent

OLEDs. Among these, the complex [Ir(fpmb)2(bptz)][22] (1)

exhibits an emission of 461 nm (fpmbH ¼ 1-(4-fluorophenyl)-

2,3-dihydro-3-methyl-1H-benzo[d]imidazole, bptzH ¼ 4-tert-

butyl-2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine).

For seeking potentially blue-emitting complexes, we theoreti-

cally design a new molecule [(fpmb)2Ir(bptz)] (2). Success in

the development of efficiently blue emitters depends to a great

extent on the knowledge of the nature of the emissive excited

states.[23] To that end, density functional theory (DFT) in this

article is applied to study the electronic effects of different

ligands and substituents in the ground and excited states

involved in the emission process. To date, in addition, first-prin-

ciple theoretical analysis on phosphorescent behavior of iridium

complexes has recently become a realistic proposition in the

framework of the time-dependent density functional theory

(TD-DFT).[23–27] Accordingly, the aim of this article is to calculate

the optical phosphorescence properties of (fpmb)xIr(bptz)3-x(x ¼ 1–2). Implementing TD-DFT method we want to present

relationship between features of electronic structures and pho-

tophysical properties of including the radiative rate constant

kr, the spin-orbital coupling (SOC) value, DE (S � T), and the

square of the SOC matrix element (\WS1.HSO.WT1[2).

Computational Detail

The ground-state and the lowest-lying triplet excited-state

geometries for each complex were optimized by the density

functional method (DFT)[28] with Becke’s LYP (B3LYP) exchange-

correlation functional[29] using the Gaussian 09 package.[30] The

B3LYP functional was used throughout with a fairly large basis

set: a Stuttgart relativistic small-core effective potential[31] for irid-

ium with its basis augmented by an f polarization function with

an exponent of 0.98.[32] There were no symmetry constraints on

these complexes. Considering large numbers of electrons, the

LANL2DZ basis set[33] was thus used on the iridium atom, while

the 6-31G (d) basis set was used on nonmetal atoms in the gra-

dient optimizations. A relativistic effective core potential on irid-

ium replaces the inner core electrons, hence leaving the outer

core 5s25p6 and 5d6 as the valence electrons of Ir (III). Thus, the

basis sets were depicted as Ir (8s6p3d/3s3p2d), C, N (10s5p/

3s2p), and F (10s4p1d/3s2p1d), and H (4s/2s).

To obtain the absorption and emission behavior of each

complex, TD-DFT calculations using the B3LYP functional with

the same basis set, associated with the polarized continuum

model[34] in dichloromethane (CH2Cl2) media, are performed at

the S0 and T1 geometries, respectively.

Compositions of molecular orbitals in terms of the constituent

chemical fragments are measured via the AOMix program.[35–37]

For the characterization of the Highest Occupied Molecular

Orbital (HOMO) � x! Lowest Unoccupied Molecular Orbital

(LUMO) þ y transitions as partial charge transfer (CT) transitions,

the CT character can be expressed as the following formula:

CTðMÞ ¼ %ðMÞHOMO� x �%ðMÞLUMOþ y

where %(M)HOMO � x and %(M)LUMO þ y presents electronic

densities on the metal in HOMO � x and LUMO þ y, respec-

tively. If the excited state, for example, S1 or T1, is formed by

more than one one-electron excitation, then the metal

CT character of this excited state is defined as a sum of CT

characters of each participating excitation, i ! j:

CT1ðMÞ ¼Xi;a

½CIði ! jÞ�2ð%ðMÞi �%ðMÞiÞ

where CI (i ! j) is denoted as the appropriate coefficients of the

Ith eigenvector of the CI matrix. Consequently, one can very

effectively use the MO compositions in terms of fragment orbital

contributions to analyze the nature of electronic transitions.

Results and Discussion

Geometries in the ground state S0 and triplet

excited state T1

The sketch drawing of 1 and 2 is presented in Scheme 1 and the

optimized ground-state geometrical structures for them are

shown in Figures 1a and 2a. To get a better understanding of

their molecular structures, the optimized geometry parameters for

the S0 and T1 states of investigated complexes are tabulated in

Table 1.

As displayed in Figure 1a, complex 1 in the S0 state shows

the slightly distorted octahedral geometry with two cyclome-

talated NHC fpmb ligands and one 2-pyridyl triazolate bptz

chelate surrounding the iridium metal center. We observe that

the benzimidazolate segment of the fpmb is located in the

mutual trans orientation, whereas the phenyl rings substituted

by a fluorine atom reside in the cis disposition, accordingly

exhibiting the typical configuration that was reported in many

other heteroleptic complexes.[7,38] According to the calculated

results in Table 1, they clearly indicate that the bonding

distance of Ir-C2 (2.027 A) is considerably longer than that of

Ir-C4 (2.021 A), which can be interpreted as the result of the

trans effect imposed by the cyclometalated bptz ligand.

Because the pyridyl and triazolate fragments of the bidentate

bptz chelate are located at the trans-disposition of two phenyl

groups substituted by a fluorine atom, respectively. At the

same time, a CF3 substituent anchored in the triazolate

Scheme 1. Molecular structures of Ir (III) complexes 1 and 2.

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Journal of Computational Chemistry 2012, 33, 1038–1046 1039

fragment is supposed to be a better electron acceptor; the net

result is to further lower electron delocalization between the

phenyl ring and the central iridium atom, and elongate the

bond length of Ir-C2. On contrary, an indirect induction effect

via a tert-butyl group makes electron delocalization from the

pyridyl transmit through the iridium ion, and extends to the

trans-orientated phenyl group; the net result is to the notably

reduced distance of Ir-C4. Moreover, it is notable that the aver-

age metal-carbon bonding distance of the carbene (2.026 A

for Ir-C1, 2.027 A for Ir-C2, 2.034 A for Ir-C3, and 2.027 A for

Ir-C4, respectively) is remarkably shorter than the respective

metal-N distances associated with the neural nitrogen donor

(2.128 A for Ir-N1 and 2.203 A for Ir-N2). Parallel to other inves-

tigations,[21,39] this observation again supports that the NHC

chelate is capable to afford a much greater ligand-to-metal

dative interaction, as compared with bptz ligand.

As depicted in Figure 2a, complex 2 also reveals a distorted

octahedral geometry around the Ir atom with two cyclometa-

lated bptz ligands and one fpmb NHC chelate. The difference

between 1 and 2 lies in that the bptz ligands of 2 adopt a

mutually eclipsed configuration with the nitrogen atoms N2

and N4 residing at the trans sites. Table 1 reports that the

bond lengths of Ir-N4 and Ir-N1 are apparently longer than

those of Ir-N3 and Ir-N2, respectively, thus indicating the ability

of the trans effect follows the trends of pyridyl [ benzimida-

zolate and phenyl[ triazolate.

With respect to bond angle and dihedral angle, the calcula-

tions show all three cyclometalated ligands tends to be per-

pendicular to each other because ffC1-Ir-N2 is almost equal to

90�, and dihedral angles of C1-Ir-N1-N2 and N2-Ir-C2-C1 are

also close to 90�, respectively (cf. Parameters for 1 and 2). A

detailed examination of the structural parameters presents

that bond angle of ffC1-Ir-N2 for both 1 and 2 is significantly

smaller than that of C1-Ir-N1. Such an observation is believed

to be caused by the strong electron repulsion of the ACH3

unit anchored on the benzimidazolate moiety and nitrogen

atom of triazolate, thereby modifying the coordination envi-

ronment. As such, ffC3-Ir-N2 is also smaller than C3-Ir-N1,

which has been supported by the calculations given in Table

1. Similar conclusions have been established for complexes

with piq or nazo chelates in recent literature,[40–42] for which

the electron repulsion originates from the pair of hydrogen

atoms located on the same p-conjugated system.

As compared with the molecular geometry of the S0 state,

there are some apparent variations in the T1 state. The bond dis-

tances of Ir-C1 and Ir-C2 as well as Ir-C3 and Ir-C4 are notably

longer than those of the S0 state, while Ir-N1 and Ir-N2 are

reduced by 0.044 A to 0.017 A, respectively, thus, indicates that

the interaction between metal atom and the cyclometalated

bptz ligand is enhanced in the T1 sate to some extent. However,

for 2, the bond strengths for Ir-N3 and Ir-N4 decrease in the T1state, which may have much to do with the difference of chemi-

cally coordinated environment versus that of Ir-N1 and Ir-N2.

Molecular orbital properties in the ground state

Since the frontier molecular orbitals (FMOs) is key to get a better

understanding of the optical and chemical properties of these

Table 1. Main optimized geometry structural parameters of 1 and 2 in

the ground and the lowest-lying triplet states at the B3LYP level.

Param

Complex 1 Complex 2

S0 T1 S0 T1

Bond distance (A)

IrAN1 2.128 2.096 2.111 2.091

IrAN2 2.203 2.186 2.075 2.054

IrAC1 2.026 2.050 1.993 1.995

IrAC2 2.027 2.053 2.030 2.031

IrAC3 2.034 2.054

IrAC4 2.021 2.038

IrAN3 2.064 2.070

IrAN4 2.070 2.080

Bond angle (�)C1AIrAN2 88.1 86.5 93.3 92.6

C1AIrAN1 98.1 98.0 99.5 100.0

C3AIrAN1 89.0 86.7

C3AIrAN2 100.9 99.6

N1AIrAN2 75.7 77.0 77.8 78.9

C1AIrAC2 78.9 79.0 79.4 79.3

C3AIrAC4 78.8 79.2

N3AIrAN4 78.3 78.2

Dihedral angles (�)C1AIrAN1AN2 86.0 84.5 91.3 90.7

N2AIrAC2AC1 87.2 85.5 92.3 91.7

Figure 2. a) Optimized structures of 2 in the ground states at DFT/B3LYP/

LANL2DZ level (For clarity, saturated H atoms are not shown. b) the diagram

showing the bonding characters (pink color) for HOMO or LUMO orbital.

Figure 1. a) Optimized structures of 1 in the ground states at DFT/B3LYP/

LANL2DZ level (For clarity, saturated H atoms are not shown. b) the dia-

gram showing the bonding characters (pink color) for HOMO or LUMO

orbital.



complexes, the aim of this section is to implement the detailed

examination on pertinent orbitals. Consequently, selective

HOMO and LUMO of the ground state complexes are presented

in Figure 3. Furthermore, more-selected information on FMOs of

1 and 2 is collected in Table 2.

As to complex 1, Table 2 indicates that the HOMO orbital of

1 mainly resides on 5dr AO of the center iridium atom (35.8%)

and the p orbital of dfbmb ligand (61.1%). We note that the

fluorine atom anchored in the phenyl ring of fpmb (b) chelate,

being a combination of electron donating effect of the ACH3

attached to benzimidazolate fragment, intensely withdraws

electron densities moving toward this phenyl ring, which lead

to the HOMO has strong bonding characters for the C4-C5

and C6-C7 linkage (see Fig. 1b). Similar situation is also found

in the HOMO composition of fpmb (a). With respect to the

LUMO, it is localized mostly on the p* orbital of the cyclometa-

lated bptz chelate. The synthesis effect of a tert-butyl substitu-

ent and one CF3 group in bptz (a) moiety is that the strong

bonding properties for C9-C8 and C11-C10 are observed. As

for complex 2, the HOMO of it extends over the p orbital of

fpmb (a) ligand (77.6%) with a large contribution (18.7%) from

5dr AO of metal atom. For HOMO orbital, due to the existence

of a fluorine atom and a ACH3 group in the fpmb (a) moiety,

the bonding character is observed in C1-N5, C2-C3, and C5-C4

(see Fig. 2b). When it comes to the LUMO of 2, it is mainly

composed of the p orbitals of both of bptz (a) and bptz (b)

ligands. Similarly, the LUMO also has the strong bonding char-

acters for C8-C9, C10-C11, and C6-C7 linkages.

As depicted in Figure 3, the HOMO and LUMO energy levels

for the complex (fpmb)2Ir(bptz) (1) are equal to �5.67 and

�1.41 eV, respectively. For 2, its HOMO and LUMO energy

levels are reduced to �6.14 and 1.66 eV, respectively. As com-

pared with 2, the enhanced HOMO and LUMO energy levels

for 1 can be reasonably interpreted as

the result of the introduction of two

cyclometalated NHC ligands, which has

an excessively large ligand-centered (LC)

p–p* energy gap than that of bptz che-

late (cf. 1 and 2 in Table 3). However, it is

notable that the HOMO–LUMO energy

gaps for 1 and 2 are in a qualitative man-

ner of 2 [ 1. Such a result may result

from the difference in the degree of

increasing the respective HOMO and

LUMO energy levels. Similar observation,

that the introduction of saturated r-bondmethylene groups inserted in the NHC

ligand leads to the difference in the

increase of the HOMO and LUMO energy

levels, is also done by Xie and co-

workers.[21] Consequently, in the current

system, it seems that the net result of the

change of the cyclometalated ligands

from the main (denoted as the fpmb in

1) to the ancillary ligand (denoted as the

fpmb in 2) is to reduce the HOMO–LUMO

energy gap.

Electronic UV-visible absorption

in CH2Cl2 media

The absorption spectrum in CH2Cl2 solu-

tion has been calculated by TD-DFT

Figure 3. Presentation of the energy levels, energy gaps and orbital compo-

sition distribution of the HOMO and LUMO for 1 and 2. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Table 2. Molecular orbital compositions in the ground state for 1 and 2 at DFT/B3LYP level

(fpmb(a)and fpmb(b) are denoted as the fpmb ligands containing C1 and C2 atoms, C3 and C4

atoms, respectively; bptz(a)and bptz(b) are denoted as the bptz ligands containing N1 and N2

atoms, N3 and N4 atoms, respectively.

Composition

Orbital Energy (ev) Ir fpmb(a) bptz(a) fpmb(b) bptz(b) Characteristics

1

L þ 6 0.02 6.5 65.6 9.0 18.9 p* (fpmb)

L þ 5 �0.14 1.8 16.3 0.7 81.3 p* (fpmb)

L þ 4 �0.23 1.7 81.9 1.5 14.9 p* (fpmb)

L þ 3 �0.73 6.5 6.3 3.8 83.4 p* (fpmb)

L þ 2 �0.80 8.1 80.4 5.2 6.4 p* (fpmb)

L þ 1 �0.91 2.1 2.7 92.6 2.6 p* (bptz)L �1.41 5.2 2.0 90.2 2.6 p* (bptz)H �5.67 35.8 23.8 3.1 37.3 d (Ir) þ p (fpmb)

H � 1 �6.01 14.8 45.4 4.0 35.8 d (Ir) þ p (fpmb)

H � 2 �6.33 34.5 15.4 23.6 26.6 d (Ir) þ p (fpmb/bptz)

H � 3 �6.42 28.6 29.5 7.2 34.6 d (Ir) þ p (fpmb)

H � 4 �6.58 29.2 50.7 8.2 11.9 d (Ir) þp (fpmb)

H � 5 �6.81 3.0 3.0 53.8 40.1 p (bptz/fpmb)

2

L þ 4 �0.88 2.9 6.4 88.8 1.8 p* (bptz)L þ 3 �0.99 3.8 25.6 1.2 69.4 p* (fpmb)

L þ 2 �1.03 5.2 55.6 7.5 31.6 p* (fpmb/bptz)

L þ 1 �1.57 6.3 1.5 22.4 69.8 p* (bptz)L �1.66 4.4 2.3 71.2 22.2 p* (bptz)H �6.14 18.7 77.6 2.1 1.7 d (Ir) þ p (fpmb)

H � 1 �6.70 37.3 6.8 24.5 31.4 d (Ir) þ p (bptz)

H � 2 �6.87 34.2 41.4 14.4 10.0 d (Ir) þ p (fpmb/bptz)

H � 3 �6.89 20.3 54.8 12.3 12.6 d (Ir) þ p (fpmb/bptz)

H � 4 �7.31 12.1 24.2 34.3 29.4 d (Ir) þ p (bptz/fpmb)



method for each complex on the basis of the optimized

group-state geometries, and simulated Gaussian-type absorp-

tion curve in CH2Cl2 media is plotted in Figure 4. Furthermore,

key transition information, such as vertical excitation energies,

oscillator strengths (f [ 0.0500), dominant configurations

(with larger CI coefficients), and assignments, is summarized in

Table 3. It is not necessary to assign the higher transition

states for 1-2 since their orbital compositions are rather com-

plicated due to the large molecular frameworks of the current

system; larger molecular frameworks may render our assign-

ments meaningless. Hence, as showed in Table 3, the calcu-

lated results of UV/vis absorption spectra for 1 reproduce its

experimental data well.

With respect to the lowest-lying

absorption, Table 3 reports clearly that

the S0 ! S1 transitions for both 1

(361 nm) and 2 (335 nm) are mainly con-

tributed by the HOMO ! LUMO electron

configuration. Since the HOMO for 1 and

2 is delocalized on the AO of Ir ion and

the p orbital of fpmb chelate, their LUMO

is located at the bptz ligand. Thus, the

vertical HOMO ! LUMO excitation

includes the spin-allowed MLCT transition

in the singlet manifold overlapped by a

large dose of LC p–p* transition. In addi-

tion, the order of the hypsochromic shift

follows the trend of 2[1.

As supported by the aforementioned

FMOs analyses, the absorption bands

ranging from 300 to 365 nm with their

molar extinction coefficient e of smaller

than 2.0 � 104 M�1 cm�1 can be mainly

attributed to the tail of the MLCT transi-

tion in the singlet manifold overlapping

with the a certain amount of the LLCT

(pfpmb ! p*bptz) transition. Parallel to

complex 1, as for 2, the broad range of

relative weaker absorption spanning the 300 to 340 nm region

is contributed by the LLCT (pbptz ! p*bptz) transition, being

mixing with a certain amount of contributions from the spin-

allowed MLCT in the singlet manifold.

Spectacularly, 1 reveals a highest absorption with a peak

wavelength at 277 nm, for which the molar extinction coeffi-

cient e is greater than 3.0 � 104 M�1 cm�1. A full examination

of the character of each transition highlights this fact that the

intense absorption can be contributed from the configuration

of HOMO-3 ! LUMO (75%), thus being characterized as

MdpLp*(bptz)CT and Lp(fpmb)Lp*(bptz)CT in the singlet manifold. As

for the case of 2, the highest absorption band located at

273 nm originates mainly from the contributions from

HOMO�3 ! LUMO þ 1 (52%) and HOMO � 4 ! LUMOþ1

(23%) electron transition configurations. As such, the absorp-

tion peak, as supported by the above FMOs analyses, is

assigned to the mixed character of MLCT, LLCT, and a certain

amount of ILCT transitions.

Phosphorescence spectra in CH2Cl2 media

On the basis of the triplet excited-state geometries calculated

by TD-DFT method, we obtained the emission spectra of 1

and 2 in CH2Cl2 solution, and the calculated results are sum-

marized in Table 4. Figure 5 depicts the selected FMOs

involved in the lowest-lying emission transitions of all exam-

ined complexes. And, all key orbital components, characters

and the corresponding assignments of each transition are

given in Table 5.

The titled complexes 1and 2 reveal their emissions with

maxima at 504 and 516 nm, respectively. It is not difficult to

find out that the calculated emission wavelength of 1 is well

Table 3. Calculated absorption of 1 and 2 in CH2Cl2 media at TD-DFT/B3LYP level.

Entry State

Oscillator

strengths E/nm (eV) Main Configurations Assignments Exptl.[22]

1 S1 0.0003 361 (3.43) H! L (96%) MLCT/LLCT 363

S2 0.0826 320 (3.87) H � 1! L (82%) MLCT/LLCT 316

S7 0.0744 292 (4.25) H � 3! L (75%) MLCT/LLCT 298

S8 0.0567 285 (4.35) H � 1! L þ 2 (63%) MLCT/ILCT

H � 1! L þ 1 (19%) MLCT/LLCT

S11 0.1399 277 (4.48) H � 1! L þ 3 (76%) MLCT/ILCT

S12 0.0515 270 (4.59) H � 5! L (47%) LLCT

H � 2! L þ 1 (28%) MLCT/LLCT

S13 0.0713 269 (4.60) H � 2! L þ 2 (63%) MLCT/LLCT/ILCT

S16 0.0620 265 (4.68) H � 5! L (24%) LLCT

H � 2! L þ 1 (33%) MLCT/LLCT

S17 0.1664 262 (4.79) H! L þ 6 (33%) MLCT/ILCT

H � 2! L þ 3 (15%) MLCT/LLCT/ILCT

H � 3! L þ 2 (11%) MLCT/ILCT

2 S1 0.0121 335 (3.70) H! L (94%) MLCT/LLCT

S3 0.1435 320 (3.87) H � 1! L (91%) MLCT/ILCT

S13 0.0525 275 (4.52) H � 4! L þ 1 (47%) MLCT/LLCT/ILCT

H � 3! L þ 1 (20%) MLCT/LLCT

S14 0.1629 273 (4.53) H � 3! L þ 1 (52%) MLCT/LLCT


S15 0.0981 267 (4.64) H � 1! L þ 3 (66%) MLCT/LLCT

H � 1! L þ 2 (22%) MLCT/LLCT

S16 0.0805 262 (4.74) H � 1! L þ 4 (42%) MLCT/ILCT


Figure 4. Simulated absorption spectra of 1 and 2 in CH2Cl2 media with

the calculated data at the TD-DFT/B3LYP/LANL2DZ level. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.com.]



consistent with the experimental value given by Chang and

co-workers.[22] In addition, the emission wavelengths of 1 and

2 follows this hypsochromic shift trend of 1 [ 2. Such a blue

shift order may originate from the change from the ancillary

(2) to main (1) ligands; a cyclometalated bptz (b) moiety in 2

is replaced with one NHC ligand dfpmb (b) in 1. To get a

better understanding of the impact imposed by the switch of

the chelating ligand from the main to ancillary on transition

nature, an in-depth analysis on FMOs of the emissive T1 state

should be raised. Table 5 depicts that the HOMO of 1 dwells

mainly on the AO orbital of iridium ion (38.5%) and p orbitals

of both the fpmb (a) and fpmb (b) ligands (19.8, 37.0%,

respectively). Similarly, the HOMO � 2 is located on the center

metal atom (43.1%), the fpmb (a) (13.7%) and the fpmb (b)

(26.8%), being mixing with a small amount of contribution

from bptz (a) moiety (16.4%), and the HOMO � 5 orbital of

the complex 1 primarily resides on the p orbitals of the bptz

(a) (56.3%) and fpmb (b) (37.1%) chelates. Thus, the investiga-

tion on these orbitals involved in the T1 !S0 electron transi-

tion arrives at the conclusion that the emissive transition of

504 nm for 1 is mainly affected by the cyclome-

talated fpmb (b) to certain extent, and that the

replacement of the NHC ligand fpmb (b) in 1

with the bptz (b) in 2 inevitably leads to the red

shift of emission wavelength. With respect to the

designed 2, we notice that the HOMO � 1 is

composed of the metal d orbital (30.5 %) and porbital for the bptz ligand (60%), while the LUMO

delocalizes mainly on the bptz ligand.

Table 4, together with Figure 5, reflects that

the 504 nm emission of 1 is contributed by H !L (71%), H � 2 ! L (11%), and H � 5 ! L (12%)

transition configurations, which can be assigned

to 3MdpLp*(bptz)CT/3Lp(fpmb)Lp*(bptz)CT,

3Lp(fpmb)Lp*(bptz)CT and3MdpLp*(bptz)CT/

3Lp(fpmb)Lp*(bptz)CT characters, respectively. As for

2, the 516 nm emission has a great contribution from the coexis-

tence of H � 1 ! L (86%) and H � 4 ! L (19%), which can be

reasonably assigned to the characteristics of 3MdpLp*(bptz)and 3MdpLp*(bptz)/

3Lp(fpmb)Lp*(bptz)/3Ip(fpmb)Lp*(bptz)CT (cf. 1 and 2

in Fig. 5).

Prediction of the radiative rate constant krfor [(fpmb)2Ir(bptz)] and [(bptz)2Ir(fpmb)]

and evaluation of SOC

The admixture of emissive singlet states into the lowest triplet

state, due to the effect of heavy metal atom, gives rise to the

phosphorescence of transition metal complexes. Therefore, it

seems that the radiative capability of the lowest triplet T1 state

stems from the transition dipole moments of the singlet Snstates.[43–45] The radiative rate constant (kr) of phosphores-

cence for transition metal complexes, in theory, can be

deduced from the transition dipole moment of the perturbed

triplet state through the use of the following formula:

kr ¼ 16p3 � 106E3n3

3he0jMj2 (1)

where n, h, e0, and E represents the refractive index of the me-

dium (denoted as CH2Cl2 solution in the current system),

Table 4. Phosphorescent emission of 1-2 in CH2Cl2 solution under the TD-DFT calculation,

together the experimental values of 1.[a]

Entry State kmax [nm] Configurations Assignments MLCT [%] Exptl[22]

1 T1 504 H! L (71%) 3MLCT/3LLCT 13.6 461

H � 5! L (12%) 3LLCT

H � 2! L (11%) 3MLCT/3LLCT

S1 361 H! L (96%) MLCT/LLCT 14.3 356

2 T1 516 H � 1! L (86%) 3MLCT 21.6

H � 4! L (19%) 3MLCT/3LLCT/ 3ILCT

S1 335 H! L (94%) MLCT/LLCT 22.7

[a] Some transitions were omitted due to its small contribution (\10%).

Figure 5. Singlet electron emission of T1 ! S0 transition for 1 (504 nm)

and 2 (516 nm), calculated at TD-DFT/ B3LYP level in CH2Cl2 solution.

Figure 6. Phosphorescence radiative rate as a function of the (1/DE (S � T))

value for the complexes investigated.



Planck’s constant, vacuum permittivity, and the emission

energy, respectively. In addition, as revealed by the first-order

perturbation theory, the transition dipole moment from the tri-

plet T1 state to the ground Sn state can be defined as this

expression, that is,

MT ¼Xn

hwTjHSOjwSni3ET �1 ESn

MSn (2)

where the meanings of wS and ES are eigenfunctions and

eigenvalues of the Hamiltonian without SOC (HSO), respec-

tively. And, the symbol of MSn means the transition dipole

moment from the Sn state to the S0 state. Consequently, the krconstant of the phosphorescence primarily depends upon the

following factors: SOC matrix element, the energy gap

between T1 and Sn, and Ms.

Thus, the approximate evaluation of kr can be depicted as

follows:[45–47]

krðT1Þ ¼ n3E3T1:5

hSjHSOjTiES � ET

� �2

� fsEs

(3)

where hSjHSOjTi and HSO present the SOC matrix element in S1! T1 transition and Hamiltonian with SOC, respectively, the

emission energy E and the matrix element are in cm�1, and n,

fs, ES, and ET are the refractive index of the medium, oscillator

strength in vacuo, the energy of the singlet and triplet states,

respectively.

In addition, among the SOC elements between Sn (dyz !p*) and triplet sublevels of T1 (dxy ! p*), only the element

involving T1, y rather than T1,x and T1,z has a nonzero value.

Accordingly, only considering the element between spin-

orbitals involving f(¼ dxy) and f(¼dyz) indicated by eq. (9) in

Ref. [48] or eq. (67) in Ref. [49], the one-center SOC element

can be simply evaluated as:

S2jHSOjT1;y� � ¼ 1

2ðhCdxyda xy jHSOjCdyzdb yzi

� hCdxydb xy jHSOjCdyzda yziÞ¼ 1

2fIr�5dCdxyCdyz ð4Þ

With respect to the eq. (4), fIr-5d presents the one-

electron SOC constant of the 5d electron of Ir ion,

and Cdxy and Cdxz represents the coefficients of the 5d

orbital related to HOMO and HOMO � 1, respectively.

Furthermore, theoretical values of fIr-5d ¼ 4430 cm�1

for the Ir (III) ion[50,51] is also used in the current arti-

cle. We could thus evaluate the SOC value by deduc-

ing the parameters in eq. (4) through the calculations

of TD-DFT method, together with the spectroscopic

measurement on the Eem value (the maximum wave-

length of emission in cm�1). Finally, the phosphores-

cence mechanism is then confirmed by assessing the

kr value using a crude approximation of the model

of the aforementioned one-center SOC element,

together with the use of eqs. (3) and (4).

As indicated by the calculated results in Table 6,

it obviously reveals that the designed (bptz)2Ir(fpmb) (2) pos-

sess relatively larger SOC values (1: 864, 2: 694, unit: cm�1) as

compared with that of complex (fpmb)2Ir(bptz) (1). As com-

pared with that of 2 (cf. 3MLCT and MLCT(%) for 1 and 2 in Ta-

ble 4), the relatively low %MLCT of both S1 and T1 states for 1

indicates the decreasing level of dp orbital participation, which

in part results in the lower SOC value. Another factor account-

ing for the smaller SOC value may be attributed to the switch

of the cyclometalated ligand from fpmb (b) in 1 to bptz (b) in

2; the latter possess a strong induction effect imposed by two

tert-butyl groups. One can thus envisage electron delocaliza-

tion from the tert-butyl pyridyl ring, transmitting through the

center iridium atom, and extending to the triazolate moiety.

Table 5. Contribution of each constituent to the frontier orbitals for complexes 1 and

2 in the threefold excited state (fpmb (a) and fpmb (b) are denoted as the fpmb

ligands containing C1 and C2 atoms, C3 and C4 atoms, respectively; bptz (a) and bptz

(b) are denoted as the bptz ligands containing N1 and N2 atoms, N3 and N4 atoms,

respectively.

MO

Composition

CharacteristicsIr fpmb(a) bptz(a) fpmb(b) bptz(b)

1

L 4.1 1.4 92.6 1.9 p* (fptz)H 38.5 19.8 4.6 37.0 d (Ir) þ p (fpmb)

H � 1 12.0 48.0 2.5 37.5 d (Ir) þ p (fpmb)

H � 2 43.1 13.7 16.4 26.8 d (Ir) þ p (fpmb/bptz)

H � 3 32.2 27.4 8.9 31.4 d (Ir) þ p (fpmb)

H � 4 27.7 44.7 11.1 16.6 d (Ir) þ p (fpmb/bptz)

H � 5 1.2 5.3 56.3 37.1 p (bptz/fpmb)

2

L 3.9 1.4 92.5 2.2 p* (bptz)H 21.4 73.2 3.5 1.9 d (Ir) þ p (fpmb)

H � 1 30.5 9.9 52.0 7.6 d (Ir) þ p (bptz)

H � 2 40.1 39.0 6.7 14.2 d (Ir) þ p (fpmb/bptz)

H � 3 29.4 60.2 4.8 5.7 d (Ir) þ p (fpmb)

H � 4 14.3 18.8 33.2 33.7 d (Ir) þ p (fpmb/bptz)

Table 6. Data used for the measurement of Phosphorescence radiative

rate constants and the evaluation of SOC value.

1 2

T1E/eV[a] 2.46 2.40

Cdxy[b] 0.55 0.50

S2 S1E/eV[a] 3.87 3.70

Cdxy[b] 0.57 0.78

F[a] 0.0826 0.0121

DE (S1-T1)/cm�1 11361 10475

SOC(S1-T1)/cm�1[c] 694 864

<WS1.HSO.WT1>210�4(eV2) 74.1 115.0

kr/s�1(n ¼ 1.42)[d] 1.47 � 105 6.56 � 105

kr, obsd/s�1(CH2Cl2)

[e] 6.00 � 105

U[e] 5.00 � 10�4

[a] Obtained from the calculation at the TD-DFT/B3LYP/LANL2DZ level.

[b] Coefficient of the natural atomic orbital of Ir 5d in the HOMO (T1)

or HOMO � 1 (S1) obtained from NBO analysis. [c] Absolute value of

the spin-orbit coupling matrix element calculated from eq. (4). [d] Value

calculated via the eq. (3). [e] Quantum yield (U) and radiative rate con-

stants kr given by the Ref. [22].



And, the ACF3 group anchored in triazolate is believed to be

a better electron acceptor, further promoting this process. The

net effect of two factors is to enhance metal-ligand interac-

tions as well as to increase the electron density at the iridium

atom and hence more MLCT character. It is notable that the

increase in %MLCT renders more metal dp contribution, thus

enhancing the SOC value. Furthermore, the HOMO (T1) and

HOMO � 1 (S1) in the 1-2 have appreciable contributions from

5d orbitals, respectively. This leads to nonzero SOC matrix ele-

ments \WS1.HSO.WT1[. Thus, the 11th column of Table 6

reports that the square of the SOC matrix element, that is,

\WS1.HSO.WT1[2, is 71.4 for 1, 150.0 for 2, (unit: 10�4 eV2),

respectively. Secondly, the energy difference between S and T

(DE (S � T)) for 1-2 remarkably increases, which leads to the

switch of transition characters from L ! H/H � 2/H � 5 to L

! H � 1/H � 4 transitions in the triplet state. Consequently,

the combination of relatively higher SOC value and smaller DE(S � T) indicates 2 may have the higher kr value, as compared

with 1. Radiative decay rate constants kr value are also pre-

dicted for 1 and 2 (1: 1.47 � 105, 2: 6.56 � 105), and our cal-

culations again support the conclusion given by the previous

literature49 that the evaluation based on one-center spin-orbit

coupling almost reproduces the magnitude of kr values of the

present Ir complexes. Furthermore, deduced from the experi-

mental data listed in Table 6, another remark worthy to men-

tion is that the phosphorescence quantum yield of complex 1

is rather low (5.00 � 10�4). As such, the nonradiative quench-

ing rate for 1 is very fast, as supported by 1.2 � 109 s�1 of its

experimental value. In fact, the previous literature[23] have

demonstrated that the\WS1.HSO.WT1[2 determines the rate of

the T1 ! S0 nonradiative quenching. The \WS1.HSO.WT1[2 for

(fpmb)2Ir(bptz) (1) is considerably large, hence this article also

predicts this nonradiative rate for 1 should be much high.

Eventually, it is necessary to understand the relationship

between the measured kr and the splitting DE (S � T). For a

better visualization of such relationship, Figure 6 is plotted. It

summarizes the dependence of the measured kr on the split-

ting DE (S � T). As presented in Figure 6, it indicates clearly

that smaller splitting value, being usually observed in the Ir

(III) system, leads generally to the relatively higher radiative

rate. Consequently, we also confirm the hypothesis[52,53] that

the 3MLCT character has an effect on the splitting DE (S � T),

which in turn controls the radiative rate kr of the present Ir (III)

complexes. Comparing 1 with 2, moreover, it is easy to

observe that the change of the cyclometalated chelate from

the main (denoted as fpmb in 1) to ancillary (denoted as fpmb

in 2) ligand may exert a certain influence on tuning the split-

ting DE (S � T).

Conclusions

We have investigated the electronic structures and optoelec-

tronic properties of complexes (fpmb)xIr(bptz)3-x (x ¼ 1–2)

((fpmb)2Ir(bptz) (1) and (bptz)2Ir(fpmb) (2), respectively),

which involve the usage of the 2-pyridyl triazolate bptz chro-

mophores substituted by a tert-butyl group and a strong-field

NHC ligand fpmb. The mechanism of low phosphorescence

yields in 1, and the evaluation of the radiative rate constant krfor 2 are also studied in this article. Our calculations on the

molecular structures in the S0 state support that bond angle

of ffC1-Ir-N2 for both 1 and 2 is significantly smaller than that

of C1-Ir-N1. Such an observation is believed to be caused by

the strong electron repulsion of the ACH3 unit anchored on

the benzimidazolate moiety and nitrogen atom of triazolate,

thereby modifying the coordination environment.

In addition, the analysis of FMOs points out that the HOMO

of 2 extends over the p orbital of fpmb (a) ligand (77.6%)

with a large contribution (18.7%) from 5dr AO of metal atom.

For HOMO orbital, due to the existence of a fluorine atom and

a ACH3 group in the fpmb (a) moiety, the bonding character

is observed in C1-N5, C2-C3, and C5-C4. When it comes to the

LUMO of 2, it is mainly composed of the p orbitals of both of

bptz (a) and bptz (b) ligands.

As to the lowest-lying absorption, the S0 ! S1 transitions

for both 1 (361 nm) and 2 (335 nm) are mainly contributed by

the HOMO ! LUMO configuration, respectively, which can be

assigned to the spin-allowed MLCT transition in the singlet

manifold overlapped by LC p–p* transition. With respect to

phosphorescence behavior in CH2Cl2 media, complexes 1 and

2 exhibit their emissions with maxima at 504 and 516 nm,

respectively. The blue shift order follows the trend of 2 \ 1.

Furthermore, to obtain mechanism of phosphorescent behav-

ior for 1-2, we approximately measure the radiative rate

constant kr (1.47 � 105 s�1for 1, 6.56 � 105 s�1for 2) the SOC

value (694 cm�1 for 1, 864 cm�1 for 2), DE (S1 � T1), and

\WS1.HSO.WT1[2 (74.1 for 1, and 115.0 for 2, unit: 10�4 eV2).

And, we also come to conclusion that the 3MLCT character has

an effect on the splitting DE (S � T), which in turn controls

the radiative rate kr of the present Ir (III) complexes.

Keywords: density functional calculation � iridium complexes �OLEDs � carbeneHow to cite this article: Q. Cao, J. Wang, Z.-S. Tian, Z.-F. Xie, F.-Q.

Bai, J. Comput. Chem. 2012, 33, 1038–1046. DOI: 10.1002/jcc.22935

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Received: 4 October 2011Revised: 24 November 2011Accepted: 25 December 2011Published online on 21 February 2012



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