A rhenium(I) complex with indolyl-containing ligand: Synthesis, photophysical properties and theoretical studies

Inorganica Chimica Acta 387 (2012) 100–105

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A rhenium(I) complex with indolyl-containing ligand: Synthesis, photophysical properties and theoretical studies Feng Zhao a,⇑, Jie-xiu Wang a, Yi-bo Wang b a b

Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Fenglin Street Nanchang, Jiangxi 330013, PR China Key Laboratory of Guizhou High Performance Computational Chemistry, Department of Chemistry, Guizhou University, Guiyang 550025, PR China

a r t i c l e

i n f o

Article history: Received 3 August 2011 Received in revised form 31 December 2011 Accepted 3 January 2012 Available online 11 January 2012 Keywords: Rhenium(I) complex Imidazo[4,5-f]-[1,10]phenanthroline Density functional theory Absorption properties

a b s t r a c t A rhenium(I) tricarbonyl complex, [Re(CO)3(IDIMPhen)Cl] (IDIMPhen-Re) [where IDIMPhen = 2-(3-indolyl)-imidazo[4,5-f]-[1,10]phenanthroline], have been successfully synthesized and fully characterized by 1 H NMR, 13C NMR, IR, GC–MS, elemental analysis, UV–Vis and cyclic voltammetry (CV). Meanwhile, the electronic structure and spectroscopic features of IDIMPhen-Re have been investigated using the density functional theory (DFT) and time-dependent DFT methods. Based on the calculated results, the experiment data are explained in great detail. The calculated orbital energies of the HOMO and LUMO of IDIMPhen-Re are in reasonable agreement with those obtained from the electrochemical measurements. The lowest lying singlet ? singlet absorption band of IDIMPhen-Re, corresponding to the prominent absorption peak at 447 nm observed in experiments, should be assigned to the pure HOMO ? LUMO transition. The calculated IP and EA show that IDIMPhen-Re possesses the good hole-transfer ability and the balanced transport of electrons and holes is more accessible compared with its analogue Phen-Re. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The d6 transition metal complexes have been attracted mostly owing to their unique photophysical and photochemical properties [1–4]. By varying the metal–ligand combination, the properties of these complexes can be tuned with respect to their physical properties (i.e. mechanical properties, solubility, processability, and optoelectric properties). In addition, modification of the ligands (L) is a more effective means of altering the photoelectric properties than making changes in the spectator ligands (X) [5,6], and the effects of varying the nature and position of substituents on a given ligand (L) have been investigated in many literatures [7–13]. Among the most frequently studied complexes is organometallic rhenium (Re) complexes since the first reported work in the 1970s [2]. The Re-based complexes offer very rich electronic structure properties companying with eight oxidation states from formal Re(0) to formal Re(VII). This diversity of electronic states has stimulated considerable current interest devoted to photocatalysis [14], probes of biological systems [15], and solar energy conversion schemes [16]. 1,10-Phenanthroline (Phen) is a classic chelating bidentate ligand for Re(I) complex owing to its rigid planar, hydrophobic, electro-poor heteroaromatic system [17–25]. Recently, a series of Re(I) complexes based on imidazo[4,5-f]-1,10-phenanthroline li-

⇑ Corresponding author. Tel.: +86 7913805183. E-mail address: [email protected] (F. Zhao). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2012.01.001

gands (IMPhen), which has interesting properties and relatively simple synthetic procedures, has been reported [26–33]. The use of easily delocalizable heterocycle groups into phenanthrolineimidazo ligands result in the excellent photoluminescent properties. While each of these motifs presents certain advantages, there is incentive to explore additional putative binding subunits that could be used to generate novel photoluminescent properties. In this paper, we present the synthesis and characterization of a novel Re complex, [Re(CO)3(IDIMPhen)Cl] (IDIMPhen-Re), where IDIMPhen = 2-(3-indolyl)-arylimidazo[4,5-f]-1,10-phenanthroline. Here, an indole moiety, which possesses the advantage of expanded p-conjugation as well as electron donor ability, should result in significant changes in the photophysical properties (such as absorption spectra and the energy gaps). In addition, the geometrical structures of the ground state and the absorption spectral properties of IDIMPhen-Re were also calculated using density functional theory (DFT) and time-dependent density functional theory (TDDFT), and the corresponding results were compared with the experimental data.

2. Experimental 2.1. Materials and methods All chemicals were of reagent grand from commercial sources and were used without further purification. 1,10-Phenanthroline5,6-dione was prepared using a reported procedure [34].

F. Zhao et al. / Inorganica Chimica Acta 387 (2012) 100–105

2.1.1. Synthesis of 2-(3-indolyl)-imidazo[4,5-f]-[1,10]phenanthroline (IDIMPhen) 1,10-Phenanthroline-5,6-dione (0.260 g, 1.24 mmol) was mixed with ammonium acetate (0.956 g, 12.4 mmol), then dissolved in glacial acetic acid (5 mL). During stirring, indole-3-carboxaldehyde (0.180 g, 1.24 mmol) in acetic acid (5 mL) was added to the above mixture. The solution was heated to 90 °C for 4 h and quenched by water (100 mL). An aqueous ammonium solution (25%) was added to neutralize the solution to pH 7. The precipitate was collected and washed with water. The crude product was further purified by crystallization from methanol. Yield: 0.295 g (71%). 1H NMR (d, DMSO-d6, 400 MHz): 13.44 (s, 1H), 11.72 (s, 1H), 9.01–9.02 (d, 3H, J = 4.8 Hz), 8.83–8.85 (d, 1H, J = 7.2 Hz), 8.67 (d, 1H, J = 2.4 Hz), 8.23 (s, 1H) 7.83 (s, 2H), 7.53 (s, 1H), 7.25–7.26 (m, 2H, J = 5.6 Hz). 13C (d, DMSO-d6, 400 MHz): 107.10, 112.41, 119.59, 120.75, 121.96, 122.77, 123.46, 123.72, 124.19, 125.31, 125.61, 125.99, 129.66, 130.07, 136.18, 136.96, 143.48, 143.75, 147.65, 147.86, 149.33. Anal. Calc. for C21H13N5H2O: C, 71.38; H, 4.28; N, 19.83. Found: C, 71.28; H, 4.29; N, 19.39%. ESI-MS calcd for C21H13N5: 335.12, found 336.5. 2.1.2. Synthesis of IDIMPhen-Re (0.076 g, IDIMPhen (0.070 g, 0.209 mmol), Re(CO)5Cl 0.221 mmol) and 15 mL ethanol were refluxed in a flask for 9 h. After the mixture was cooled to RT, the solvent was removed in a water bath under reduced pressure. The resulting orange yellow solid was washed with methanol. Yield: 0.109 g (81%). 1H NMR (d, DMSO-d6, 400 MHz): 13.82 (s, 1H), 11.77 (s, 1H), 9.26–9.33 (m, 3H), 9.08–9.10 (d, 1H, J = 8 Hz), 8.61–8.63 (d, 1H, 5.6 Hz), 8.23 (s, 1H) 8.09–8.11 (d, 2H, 7.2 Hz), 7.53–7.54 (d, 1H, J = 5.2 Hz), 7.25– 7.27 (m, 2H, J = 7.2 Hz). 13C (d, DMSO-d6, 400 MHz): 106.45, 112.55, 121.02, 121.89, 122.99, 125.47, 126.00, 126.66, 126.82, 132.96, 136.97, 143.62, 151.38, 190.61, 198.35. FT-IR (KBr disk): mNH 3510, 1578 cm1, mCO 2021, 1916, 1899 cm1. Anal. Calc. for: C24H13N5O3(Re)ClH2O: C,43.74; H, 2.29; N, 10.63. Found: C, 43.96; H, 2.97; N, 10.82%. ESI-MS calcd for C24H13N5O3(Re)Cl: 641.05, found 640.2. 2.2. Measurements 1

H NMR spectra were performed in a Bruker AV400 MHz spectrometer, using tetramethylsilane (TMS) as internal reference. UV–Vis absorption spectra were measured using a Perkin–Elmer Lambda-900 spectrophotometer. Fluorescence spectra were determined with an F-4500 fluorescence spectrophotometer (Hitachi). The FTIR spectra were recorded via the KBr pellet method by using a Bruker V70 FTIR spectrophotometer. Contents of carbon, hydrogen and nitrogen were determined using a Vario EL III element analyzer. Mass spectra were performed on a Bruker amaZon SL ion trap mass spectrometer (ESI-MS, APCI-MS) with positive mode. Cyclic voltammetry measurement was performed in a one-compartment cell with a model 263A potentiostat/galvanostat (EG&G Princeton Applied Research) under computer control. The voltammogram was obtained in acetonitrile solutions with 0.1 M sample and 0.1 M Bu4NBF4 as the supporting electrolyte. A polished Pt plate was used for the working electrode, and a polished Pt mesh as the counter electrode. The voltammogram was recorded versus a Ag/AgCl electrode at a scan rate of 100 mV/s. Prior to each electrochemical measurement, the solution was purged with nitrogen for 10–15 min to remove the dissolved O2 gas. In order to precisely calculate the energy levels of the HOMO and the LUMO, the offset potentials are determined by the point of intersection of the tangential lines near the beginning part of the redox peaks.

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2.3. Computational details The geometrical structure of the ground states of IDIMPhen-Re was optimized by the density functional theory (DFT) method with B3LYP functional [35,36]. On the basis of the optimized ground state geometry structures, the absorption spectral properties in DMSO media were calculated by time-dependent DFT (TDDFT) approach associated with the polarized continuum model (PCM) [37,38]. The 6-31G⁄ basis set was employed on C, H, N, O, and Cl atoms, and LANL2DZ basis set was adopted on Re atom. All calculations have been performed using the GAUSSIAN 09 [39] program package.

3. Results and discussion 3.1. Synthesis The synthetic pathway of the Re-complex is shown in Scheme 1. IDIMPhen was prepared by a condensation reaction between 1,10-phenanthroline-5,6-dione and indole-3-carboxaldehyde [40]. The complex IDIMPhen-Re was prepared by the modified literature procedures [41]. The structures of IDIMPhen and IDIMPhen-Re have been characterized and identified (see Figs. S1–S6 of Supporting information).

3.2. Photophysical properties The UV–Vis absorption spectra of IDIMPhen-Re are shown in Fig. 1a. For comparison purposes, the UV–Vis absorption spectra of the model complex Phen-Re are also shown in Fig. 1b. For IDIMPhen-Re complex, the absorption spectrum exhibits an intense ligand-based p ? p⁄ absorptions band at 278 nm with one shoulder peak being at lower-energy side, and a broad weaker metal-to-ligand charge transfer dp(Re) ? p⁄(N–N) (1MLCT) transitions band at 447 nm. The p ? p⁄ absorptions band was blueshifted compared to that of Phen-Re complex localized at 306 nm. IDIMPhen ligand possesses a more delocalized p system, which increases the energy levels of p ? p⁄ excitation transition of the interligand or intraligand. On the other hand, the lowest 1MLCT transitions band of IDIMPhen-Re was red-shifted due to the decrease in LUMO–HOMO gap. Fig. 2 shows the photoluminescence spectra of IDIMPhen-Re and Phen-Re complexes in DMSO media at room temperature. IDIMPhen-Re shows relative emission with a peak at 495 nm which is about a 72 nm blue shift compared with that of PhenRe. so that, large changes in the emission and electronic structure are observed by changing the ligand environment. And the optical properties of such metal complexes provide a possibility for molecular design to change the emission color and to improve the organic photovoltaic (OPV) device efficiency and aid applications such as OLEDs and organic field-effect transistors (OFETs).

Scheme 1. Synthetic route of IDIMPhen-Re complex.

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Fig. 1. UV–Vis spectra of IDIMPhen-Re and Phen-Re in DMSO.

Fig. 3. Cyclic voltammogram of IDIMPhen-Re measured in CH3CN at a scan rate of 100 mV/s. A polished Pt plate and a Pt mesh were used as the working electrode and the counter electrode, respectively. Bu4NBF4 was taken as the supporting electrolyte.

3.4. Theoretical calculations 3.4.1. Ground state geometries of IDIMPhen-Re The optimized ground state geometric structure of IDIMPhenRe is shown in Fig. 4, and selected bond lengths and angles are summarized in Table 1. Vibrational frequencies were calculated on the basis of the optimized geometries to verify that geometries represented a minimum on the potential energy surface. The N(1)– Re–N(2) bond angle is about 74.9°, indicating that Re(I) adopts a distorted octahedral coordination geometry. The optimized structural parameters are in general agreement with the experimental values of the similar structural analogues [44].

Fig. 2. Photoluminescence spectra of IDIMPhen-Re and Phen-Re complexes in DMSO media at room temperature.

3.3. Electrochemical properties Cyclic voltammetric experiment was carried out in CH3CN solution of 0.1 M Bu4NBF4 at a scan rate of 100 mV/s. Cyclic voltammogram of IDIMP-Re presented in Fig. 3 exhibit irreversible redox behaviors in CH3CN solution. The anodic wave of the Re(I) complex is associated with a ReI-based oxidation process (ReI/ReII), and the cathodic waves are associated with a ligand-based reduction process ([ReICl(CO)3(L)]/[ReICl(CO)3(L)]) [42]. From the onset oxidation potentials (Eox) and the onset reduction potentials (Ered), HOMO and LUMO energy levels as well as the energy gap (Eg) of IDIMP-Re complexes were calculated according to the equations [43]:

HOMO ¼ ðEox þ 4:40Þ ðeVÞ;

LUMO ¼ ðEred þ 4:40Þ ðeVÞ;

Eec g ¼ eðEox  Ered Þ ðeVÞ The oxidation and reduction potentials of IDIMPhen-Re were measured to be 1.09 and 0.92 V, respectively, and the energy band gap (Eg) was calculated to be 2.01 eV, the energy value of the HOMO was calculated to be 5.49 eV and the energy value of the LUMO was calculated to be 3.48 eV.

3.4.2. Frontier molecular orbital properties The frontier molecular orbital compositions of IDIMPhen-Re are shown in Table 2. The compositions of the two highest occupied orbitals (HOMO and HOMO1) of IDIMPhen-Re are similar with only marginally different compositions. HOMO and HOMO1 orbitals consist of d(Re), p(Cl) and a lesser contribution from p(CO). HOMO2 orbital predominantly consists of p(Indol) (>55%) with a considerable contribution from p(IMPhen) (>40%). HOMO3 orbitals predominantly consist of d(Re) with a lesser contribution from p(CO). The compositions of HOMO4 orbital are predominantly localized on indol moiety (>99%). The composition of the four lowest unoccupied orbitals (LUMO, LUMO+1, LUMO+2 and LUMO+3) of IDIMPhen-Re are mainly based on the IMPhen moiety with p⁄ character, but LUMO+2 and LUMO+3 orbitals also contain 31.3%, 53.4% p⁄(Indol) contributions, respectively. LUMO+4 contains 20.4% p(Re), 40.5% p⁄(CO) and 36.9% p⁄(IMPhen) contributions. The orbital energies of the HOMO of IDIMPhen-Re are calculated to be 5.41 eV, which is well in agreement with that (5.49 eV) obtained from the electrochemical measurements. The corresponding value of LUMO is calculated to be 2.47 eV, which is slightly larger deviation than the value of 3.48 eV obtained from the electrochemical data mentioned above, but this discrepancy is still an acceptable result [45]. 3.4.3. Theoretical absorption spectra Calculated absorption spectra associated with their oscillator strengths, assignment, configurations, excitation energies, and excitations with maximum coefficients are listed in Table 3. The

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Fig. 4. Optimized ground state geometric structures of IDIMPhen-Re.

Table 1 Selected parameters of the optimized geometric structures of IDIMPhen-Re in the ground state. IDIMPhen-Re Bond length (Å) Re–C1 Re–C2 Re–C3 Re–N1 Re–N2 Re–Cl

1.928 1.927 1.920 2.210 2.210 2.520

Bond angle (deg) N1–Re–N2 a

Exptla 1.928 1.921 1.981 2.188 2.191 2.500

74.9

74.1

From Ref. [44].

corresponding simulated UV–Vis absorption spectra, presented as oscillator strength against wavelength, are shown in Fig. 5. The best agreement with the experimental spectra (Fig. 1a) was observed. Fig. 5 shows that the lowest lying weaker singlet ? singlet absorption band of IDIMPhen-Re localized at about 480 nm (Region I), corresponding to the prominent absorption peak at 447 nm observed in experiments, should be assigned to the pure HOMO ? LUMO transition from the excite state 1. The HOMO of IDIMPhen-Re consists of 37.6% d(Re), 18.8% p(CO) and 40.7% p(Cl), whereas the LUMO of IDIMPhen-Re is predominantly composed of p⁄(IMPhen) at 90.0%. Thus, the lowest-lying absorption band at 480 nm for IDIMPhen-Re can be described as a [d(Re) + p(CO) + p(Cl)] ? [p⁄(IMPhen)] transition with MLCT/LLCT character. The second prominent absorption Region II of 384– 400 nm mainly originate form the mixed contributions of excite states 3 and 4. The former consists of the transitions of H ? L+1 (69.7%) with a few contribution of the H-1 ? L transitions (27.6%), and the latter predominantly consists of the transitions

of H-3 ? L (97.7%). So this absorption region can be described as [d(Re) + p(CO) + p(Cl)] ? [p⁄(IMPhen)] with MLCT/LLCT character. The pure transitions of H ? L+2 originated from the excite state 10 contribute to the 328 nm absorption of IDIMPhen-Re (Region III), which can be ascribed as [d(Re) + p(CO) + p(Cl)] ? [p⁄(IMPhen) + p⁄(Indol)] with MLCT/LLCT character. It is worth nothing that the mixed transitions of excite state 19, 20 and 25 are responsible for the absorption bands of Region IV with the transitions of H-4 ? L+1/H-6 ? L/H ? L+5/H-6 ? L+1/H-8 ? L, and the corresponding transition characters are listed in Table 3. This absorption bands are well agreement with our experimental results for the absorption bands at 278 nm. The excitation of H-4 ? L+2 is the dominant contribution to the shoulder peak at 252 nm (Region V), and it can be described as [p(Indol)] ? [p⁄(IMPhen)] with LLCT character. 3.4.4. Ionization potential (IP), electron affinity (EA) and reorganization energy In OLED devices, the balanced injection and transport of electrons and holes is a prerequisite for high performance OLED devices [46], since the emission of light originates from the relaxation of excitons produced by the recombination of electrons and holes which are injected from the cathode and the anode. Ionization potential (IP) is defined as the minimum energy necessary to bring an electron from the material into vacuum, namely, the energy difference between the neutral molecules and the corresponding cationic systems. Electron affinity (EA) is defined as the first unoccupied energy level which injected electrons from vacuum into the material would occupy, namely, the energy difference between the neutral molecules and the corresponding anionic systems. The IP and EA are usually used to assess the energy barrier for the injection of holes and electrons [47]. For the similar structure complexes, the smaller IP and larger EA will facilitate

Table 2 Frontier molecular orbital compositions (%) for IDIMPhen-Re the ground state at the B3LYP/6-31g⁄/LANL2DZ level. Orbital

Energy (eV)

Bond type

LUMO+5 LUMO+4 LUMO+3 LUMO+2 LUMO+1 LUMO

0.19 0.35 0.79 1.46 2.12 2.47

p⁄(IMPhen) p(Re) + p⁄(CO) + p⁄(IMPhen) p⁄(IMPhen) + p⁄(Indol) p⁄(IMPhen) + p⁄(Indol) p⁄(IMPhen) p⁄(IMPhen)

Contribution (%) Re

HOMO–LUMO gap (2.94 eV) HOMO 5.41 HOMO1 5.47 HOMO2 5.84 HOMO3 6.17 HOMO4 6.64 HOMO6 6.68 HOMO8 7.24

d(Re) + p(CO) + p(Cl) d(Re) + p(CO) + p(Cl) p(IMPhen) + p(Indol) d(Re) + p(CO) p(Indol) d(Re) + p(Cl) p(Cl) + p(IMPhen)

20.4

CO

Cl

40.5

37.6 34.4

18.8 15.4

68.0

28.6

IMPhen

Indol

68.1 36.9 43.6 66.8 94.5 90.0

53.4 31.3

40.5

55.9

40.7 42.5

99.5 24.2

50.9 20.7

65.6

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F. Zhao et al. / Inorganica Chimica Acta 387 (2012) 100–105

Table 3 Electronic absorptions of IDIMPhen-Re in DMSO from TDDFT calculations at the B3LYP/6-31g⁄/LANL2DZ level. Excited state 1 3 4 10 19 20 25 32

Transition

Coefficient

E (eV)/(nm)

Oscillator

Assign

Exptl (nm)

H?L H ? L+1 H1 ? L H3 ? L H ? L+2 H4 ? L+1 H6 ? L H ? L+5 H6 ? L+1 H8 ? L H4 ? L+2

0.69118(100%) 0.57610(69.7%) 0.36239(27.6%) 0.67517(97.7%) 0.66492(100%) 0.44436(48.8%) 0.30759(23.4%) 0.41583(39.2%) 0.39664(35.7%) 0.48115(54.0%) 0.50092(61.6%)

2.5844/480 3.0989/400

0.0565 0.0995

447 378

3.2270/384 3.7837/328 4.3222/287

0.0945 0.6489 0.2052

4.4112/281

0.1730

4.5967/270 4.9213/252

0.1878 0.2482

MLCT/LLCT MLCT/LLCT MLCT/LLCT MLCT/LLCT MLCT/LLCT LLCT MLCT/LLCT MLCT/LLCT MLCT/LLCT LLCT/ILCT LLCT/ILCT

310

278

of Phen-Re (7.28 eV), indicting that the hole-transporting ability of IDIMPhen-Re is better than that of Phen-Re. On the other hand, the electron-transporting ability of Phen-Re with an EA of 1.18 eV should be better than that of IDIMPhen-Re with an EA of 1.09 eV. In addition, the khole of IDIMPhen-Re is smaller than the corresponding kelectron, suggesting that the hole-transfer rate is better than the electron-transfer rate. For Phen-Re, the electrontransfer rate is better than the hole-transfer rate due to kelectron < khole. Furthermore, the difference between kelectron and khole for IDIMPhen-Re (0.04) is smaller than that of Phen-Re (0.09), which suggests that the balanced transport of electrons and holes of IDIMPhen-Re is more accessible. The knowledge of this information is very important for the understanding lightemitting diode properties and the potential optimization of OLED devices. 4. Conclusions

Fig. 5. Electronic transitions calculated at the TDDFT (B3LYP)/6-31g⁄/LANL2DZ level for IDIMPhen-Re (a) and its corresponding simulated absorption spectra in DMSO (b).

Table 4 Calculated IP, EA, HEP, and k for IDIMPhen-Re and Phen-Re.

a b

Complex

IP(v)

IP(a)

HEP

kholea

IDIMPh-Re Phen-Re

6.83 7.28 EA(v)

6.66 7.03 EA(a)

6.52 6.77 EEP

0.31 0.51 kelectronb

IDIMPhen-Re Phen-Re

1.09 1.18

1.28 1.34

1.44 1.60

0.35 0.42

khole = IP(v)  HEP. kelectron = EEP  EA(v).

In this paper, the novel rhenium(I) tricarbonyl complexes (IDIMPhen-Re), which possess imidazo[4,5-f]-[1,10]-phenanthroline ligand with indol moiety, were successfully synthesized and the corresponding photophysical and electrochemical properties were characterized. Meanwhile, the ground geometries, frontier molecular orbital properties, and absorption properties of IDIMPhen-Re were studied theoretically. The calculated orbital energies of the HOMO and LUMO of IDIMPhen-Re are in reasonable agreement with those obtained from the electrochemical measurements. The experimental absorption spectra of IDIMPhen-Re feature two prominent absorption bands with maxima at 447 and 278 nm, respectively. Theoretical calculations suggest that the former is dominated by the pure HOMO ? LUMO transition with MLCT/LLCT character, and the latter mainly originates from the mixed transition of excite state 19, 20 and 25 with the character of LLCT/MLCT/ILCT. Acknowledgment

the hole- and electron-transporting abilities. IP, EA, reorganization energy (k) hole extraction potential (HEP) and electron extraction potential (EEP) for IDIMPhen-Re and model complex Phen-Re are listed in Table 4. The charge transfer rate k can be expressed by the following formula [48]:

  k k ¼ A exp  4kb T

ð1Þ

where A is a constant under certain condition, kb is the Boltzmann constant, T is the temperature. Obviously, in order to achieve high charge transfer rate, the lower reorganization energy is necessary. IDIMPhen-Re has an IP(v) of 6.83 eV, which is smaller than that

The authors acknowledge the financial support from National Natural Science Foundation of China (No. 20903049). They thank the Guizhou University High Performance Computation Chemistry Laboratory (GHPCC) for help with computational studies. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2012.01.001. References [1] D. Donghi, G. D’Alfonso, M. Mauro, M. Panigati, P. Mercandelli, A. Sironi, P. Mussini, L. D’Alfonso, Inorg. Chem. 47 (2008) 4243.

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