Substituent and Solvent Effects on Electronic Structure and Spectral

Oct 28, 2010 - Fax & phone: +86 0357 2052468. E-mail: [email protected]. Cite this:J. Phys. .... Paul I. P. Elliott. Annual Reports Section "A" (I...
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J. Phys. Chem. A 2010, 114, 12251–12257

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Substituent and Solvent Effects on Electronic Structure and Spectral Property of ReCl(CO)3(N∧N) (N∧N ) Glyoxime): DFT and TDDFT Theoretical Studies Ting-Ting Zhang, Jian-Feng Jia, and Hai-Shun Wu* School of Chemistry and Materials Science, Shanxi Normal UniVersity, Linfen, 041004, China ReceiVed: May 16, 2010; ReVised Manuscript ReceiVed: August 23, 2010

The ground- and excited-state structures of five Re(I) halide glyoxime complexes ReCl(CO)3(N∧N) (N∧N ) glyoxime (DHG 1), dimethylglyoxime (DMG 2), cyclohexane dione glyoxime (CHDG 3), dibromoglyoxime (DBG 4), and dimethylformylgloxime (DMFG 5)) have been studied with density functional theory (DFT) and configuration interaction with single excitations (CIS) methods. Time-dependent density functional theory/ polarized continuum model (TDDFT/PCM) was carried out to predict the absorption and emission spectra in different media. The effect of substituent and solvent has been researched. It is found that electron-donating groups increase the lowest unoccupied molecular orbital (LUMO) energy resulting in the increased highest occupied molecular orbital (HOMO)-LUMO energy gap. The change leads to their absorption spectra blue shifts in the order 1 > 2 > 3, which arises from the HOMO-1 f LUMO. Just the opposite, electron-withdrawing groups lead to the spectra red shifts (5 > 4 > 1) because of the decreased HOMO-LUMO energy gap. The reorganization energy (λ) calculations show that the relatively balanceable charges transfer abilities of 2 will result in the higher efficiency of organic light emitting devices (OLEDs). In addition, both the absorption and the emission spectra display red shifts in different extents with the decrease of solvent polarity. 1. Introduction Re(I) tricarbonyl complexes have been extensively applied as photosensitizers on the solar cell,1 photocatalysts,2 luminescence probes,3 and molecular parts of supramolecules.4 The redox and spectroscopic behavior can be tuned by varying the identity of their chelate ligands. The most frequently studied compounds of this type are the general formula fac[ReX(CO)3(N∧N)], where X stands for a halogen atom or other ligand approximately axial to the heteroaromatic rings plane. In the past few years, a large number of complexes [ReX(CO)3(Rdiimine)] have been studied for their stability as well as for their interesting chemical, physical, and biological properties. For example, the photoexcitation of the complex5 [Re(CO)3(bipy)Cl] results in the conversion of CO2 f CO. Their photophysical and photochemical behavior depends on the nature of low-lying excited states, which can be controlled by structural variations of the ligand or by the medium. The replacement of X in ReCl(CO)3(X2-bipy) complexes6 with different π-conjugated species can significantly change the spectral properties of these complexes. Recently, several novel Re(I) complexes containing N∧N ligand have been extensively reported experimentally and theoretically. Howell7 et al. have synthesized and studied a series of [Re(L)(CO)3Cl] complexes using density functional theory (DFT), where L ) 3,3′-dimethylene-2,2′-bi-1,8-naphthyridine (dbn), 2,2′-bi-1,8-naphthyridine (bn), 3,3′-dimethylene-2,2′biquinoline (dbq), and 3,3′-dimethyl-2,2′-biquinoline (diq). The calculations reveal that the accepting molecular orbital lowest unoccupied molecular orbital (LUMO) in MLCT (metal to ligand charge-transfer transition) transition causes bonding changes at the interring section of the ligand. Machura et al.8 have reported some novel tricarbonyl rhemium complexes, which can be used in diagnostic and therapeutic radiopharma* To whom correspondence should be addressed. Fax & phone: +86 0357 2052468. E-mail: [email protected].

ceuticals, such as [Re(CO)3(tp)2Cl], [Re(CO)3(bpzm)Cl], and [Re(CO)3(bdmpzm)Cl] (tp )1,2,4-triazolo-[1,5-a]pyrimidine, bpzm ) bis(pyrazol-1-yl)methane, bdmpzm ) bis(3,5-dimethylpyrazol-1-yl)methane). The oxime complexes, some of which have been used for the labeling of antibodies,9 have attracted considerable interest as for their relative long lifetimes and high stability. For example, the complexes ReL(CO)3X (L ) glyoxime (DHG), dimethylglyoxime (DMG), diphenylglyoxime, 1,2-cyclohexanedione glyoxime (CHDG); X ) Cl, Br) have been synthesized and characterized.10 The glyoxime complexes with luminescent properties are of great interest in their medical and technological applications, but the spectroscopic property resulting from MLCT has not been interpreted from an electronic structure point of view. In this study, a detailed theoretical investigation of electronic structure and spectral properties of five Re(I) halide glyoxime complexes ReCl(CO)3(DHG) (1), ReCl(CO)3(DMG) (2), ReCl(CO)3(CHDG) (3), ReCl(CO)3(DBG) (DBG ) dibromoglyoxime) (4), and ReCl(CO)3(DMFG) (DMFG ) dimethylformylgloxime) (5) were carried out using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The aim of the theoretical investigation is to establish how the substituents on glyoxime and the solvents affect the spectra property, and this is interpreted from an electronic structure point of view. Then, the luminescent rule of such complexes can be obtained for future synthesizing and designing new luminescent materials with a practical application perspective. 2. Computation Methods All calculations have been performed using the Gaussian 0311 program package. The geometries of the singlet ground state and the lowest triplet state of 1-5 were optimized by the density functional theory (DFT)12 method with B3LYP functional13 and by configuration interaction with single excitations14 (CIS) methods, respectively. For excited states, the CIS method is most

10.1021/jp104458u  2010 American Chemical Society Published on Web 10/28/2010

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TABLE 1: Main Optimized Ground-State Geometry Parameters of 1-5 Together with the X-ray Crystal Diffraction Data of 1-3 1 Bond Length (Å) Re-C(1) Re-C(2) Re-C(3) Re-N(1) Re-N(2) Re-Cl C(4)-N(1) C(4)-C(5) Bond Angle (deg) N(1)-Re-N(2) Cl-Re-N(l)

3

4

5

S0

T1

exptl

S0

exptl

S0

S0

S0

1.935 1.935 1.923 2.142 2.142 2.531 1.316 1.443

1.965 1.965 1.943 2.188 2.188 2.552 1.386 1.366

1.930(9) 1.930(9) 1.894(1) 2.165(6) 2.165(6) 2.513(4) 1.298(1) 1.443(2)

1.933 1.933 1.919 2.145 2.145 2.537 1.318 1.473

1.928(1) 1.928(1) 1.903(2) 2.161(8) 2.161(8) 2.493(3) 1.290(1) 1.475(2)

1.932 1.932 1.918 2.144 2.146 2.538 1.318 1.468

1.933 1.933 1.924 2.148 2.148 2.527 1.315 1.466

1.937 1.938 1.927 2.134 2.132 2.527 1.321 1.456

72.9 84.9

72.3 83.4

2

72.5(4) 84.3(2)

similar to the Hartree-Fock (HF) method for the ground state. However, through using analytic gradients and methods properly including the effects of electronic correlation, the bond lengths, frequencies, and dipole moments at the CIS level are better than DFT, which is in line with the experimental values, and the application of the method to very large molecules is possible. On the basis of the optimized ground- and excited-state geometries, the absorption and emission properties in methanol media were calculated by time-dependent DFT (TDDFT)15 associated with the polarized continuum model16 (PCM). In the calculations, the “double-” quality basis set LANL2DZ was adopted as the basis set. This kind of theoretical approach has been proven to be reliable for Re(I) tricarbonyl complex systems.17 3. Results and Discussion 3.1. The Ground-State Geometries. The main optimized ground-state geometry parameters in the gas phase together with the X-ray crystal diffraction data of 1 and 2 are given in Table 1, and the optimized structures of 1-5 are shown in Figure 1. Vibrational frequencies were calculated on the basis of the optimized geometries of 1-5 to verify that each of the geometries is a minimum on the potential energy surface.

72.1 83.9

72.0(0) 87.5(2)

72.3 83.9

72.9 84.6

73.0 85.3

The optimized bond lengths and bond angles are in general agreement with the corresponding experimental values from Table 1. The calculated structure of 1 shows that the Re(I) adopts a distorted octahedral coordination geometry with the three carbonyls arranged in a facial configuration as expected for Re(CO)3+ compounds, and the halide is slightly tilted toward the DMG unit. The Re-C(1) and Re-C(2) bond lengths are longer than that of Re-C(3). This is attributed to the different ligand-to-metal back-bonding abilities at the axial and the equatorial positions.18 The C(4)-N(1) bonds (1.316 Å) are longer than those observed in neutral DHG (1.30 Å), and it can also be deduced from π-back-bonding to the metal,10 which presents MLCT transitions in the absorption. The bond length of Re-N is slightly shorter than the Re(I) halide bipyridine complex ReCl(CO3)(bpy).19 Moreover, the calculated Re-Cl bond length is overestimated by 0.018 Å, which is consistent with the research of other groups.20 It is attributed to the drawback of DFT arising from the dynamical correlation effects.21 Although the DHG was substituted by different groups in 2-5, they have similar geometrical rearrangements to 1 according to calculations. 3.2. Frontier Molecular Orbital Properties. The frontier molecular orbital compositions and energy levels of 1 and 3

Figure 1. Optimized ground-state geometry structure of 1-5 at the B3LYP/LANL2DZ level.

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TABLE 2: Frontier Molecular Orbital Compositions (%) in the Ground State for Complex 1 at the B3LYP/LANL2DZ Level contribution (%) orbital

energy(eV)

Re

CO

DHG

Cl

59a′′ 57a′′ 56a′

–1.1451 –2.1298 –3.9331

15.8 4.3

84.6 17.0 7.1

5.9 53.7 78.3

1.1 3.3

55a′′ 54a′ 53a′ 51a′

–6.7048 –6.8190 –7.7003 –8.0920

35.3 26.8 64.0 39.1

16.5 6.7 29.4 14.1

main bond type

HOMO–LUMO Energy Gap 2.2 39.0 46.1 10.0

28.5

Re component

π*(CO) π*(CO) + π*(DHG) π*(DHG)

7.4 dyz 1.7 dxy

d(Re) + p(Cl) + π(CO) d(Re) + p(Cl) d(Re) + π(CO) d(Re) + p(Cl) + π(CO)

29.2 dxz dxy 48.2dz2 + 13.2dxz 1.4dx2–y2 + 37.8dxy

TABLE 3: Frontier Molecular Orbital Compositions (%) in the Ground State for Complex 3 at the B3LYP/LANL2DZ Level contribution (%) orbital

energy (eV)

Re

CO

DHG

74a 72a 71a

–0.8432 –1.8496 –3.3946

17.3

75.2 50.8 5.3

19.3 76.4

70a 69a 68a 66a

–6.4192 –6.5280 –7.3712 –7.6813

35.5 31.1 62.5 34.7

16.9 8.6 29.5 12.7

Cl

main bond type

HOMO–LUMO Energy Gap 36.8 42.6 10.5

30.6

Re component

π*(CO) π*(CO) + π*(CHDG) π*(CHDG)

7.8 dxy

d(Re) + p(Cl) + π(CO) d(Re) + p(Cl) d(Re) + π(CO) d(Re) + p(Cl) + π(CO)

30.2 dyz 1.05 dz2+23.2 dxz 59.6 dx2 – y2 1.2 dz2 + 33.5 dxz

TABLE 4: Ionization Potentials, Electron Affinities, and Spin Densities for the Five Complexes (in eV) Calculated at the DFT/B3LYP Level spin density of cation (%) 1 2 3 4 5

IP(v)

IP(a)

HEP

λhole

Re

DHG

3CO

Cl

8.53 8.37 8.32 8.55 8.47

8.32 8.02 7.95 8.35 8.35

8.32 8.03 8.32 9.70 8.25

0.21 0.34 0.00 -0.15 0.22

0.60 0.60 0.59 0.59 0.59

0.03 0.05 0.06 0.04 0.04

0.07 0.07 0.08 0.07 0.07

0.30 0.28 0.27 0.29 0.30

2CH3

4CH2

2Br

2COOCH3

0.00 0.00 0.01 0.00

spin density of anion (%) 1 2 3 4 5

EA(v)

EA(a)

EEP

λelectron

Re

DHG

3CO

Cl

2.18 1.75 1.75 2.47 2.46

2.40 2.07 2.04 2.83 2.83

2.40 2.07 2.04 2.83 2.99

0.22 0.32 0.29 0.36 0.53

-0.01 -0.02 -0.02 -0.01 -0.01

0.90 0.90 0.89 0.88 0.83

0.07 0.07 0.07 0.06 0.07

0.04 0.04 0.04 0.03 0.03

are compiled in Tables 2 and 3, respectively, while those of the other complexes are shown in Tables S1-S3 (Supporting Information). Table 3 and Tables S1-S3 show that the frontier molecular orbital compositions of 2-5 are similar to that of 1. The highest occupied molecular orbital (HOMO) and HOMO-1 orbitals are contributed by d(Re), p(Cl), and very little π(CO). Therefore, the orbitals are hardly affected by the glyoxime ligand. The HOMO-2 orbital is mainly composed of d(Re) (over 60%), and HOMO-4 is composed of d(Re), p(Cl), π(CO), and π(glyoxime). Besides, the composition of the LUMO is π* orbital localized on the glyoxime moiety with more than 67% composition. However, different substituents have a direct effect on orbital energy of LUMO, which is mainly composed of glyoxime. From the energies listed, it can be observed that the electron-donating -CH3 and -(CH2)4- groups in complexes 2 and 3 increase the LUMO energy by 0.4978 and 0.5386 eV; the electronwithdrawing -Br and -COOCH3 groups in complexes 4 and 5 are just the opposite, which slightly decrease the LUMO energy by 0.1496 and 0.1904 eV. Finally, the electron-donating groups result in the increased HOMO-LUMO energy gaps in the order 2 (3.0328 eV) > 3 (3.0246 eV) > 1 (2.7717 eV), and

2CH3

4CH2

2Br

2COOCH3

0.01 0.02 0.04 0.08

the electron-withdrawing groups result in decreased energy gaps in the order 1 (2.7717 eV) > 4 (2.7690 eV) > 5 (2.6221 eV), which then lead to the change in the absorption and emission spectra. The reason is that the participation of the electrondonating or electron-withdrawing groups with the deduced or enhanced conjugation effect in the composition of the LUMO increases or decreases the energy level. The same conclusion has been obtained for other Re compleses.18 3.3. Ionization Potentials and Electron Affinities. Ionization potentials (IP) and electron affinities (EA) can explain the inter-relationship of structure and electronic behavior. In addition, they have been demonstrated to have a linear relationship to the energy levels of the HOMO and LUMO:22 HOMO ) -(Eox + 4.71) eV and LUMO ) -(Ered + 4.71) eV. Table 4 contains the IP and EA, both vertical (v, at the geometry of the neutral molecule) and adiabatic (a, optimized structure for both the neutral and charged molecule), the hole extraction potential (HEP), the electron extraction potential (EEP), the reorganization energy (λ), and the spin densities for the five complexes. The five complexes share the common features of having accessible Re-based oxidations and glyoxime-based reductions. As expected by the trend in the HOMO energy, the change of

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TABLE 5: The Absorptions of 1-5 in Methanol Calculated According to TDDFT Method Together with the Experimental Values complex 1 2 3 4

5

transition H-1 f L H-4 f L H f L+3 H-1 f L H-4 f L H-2 f L+3 H-1 f L H-4 f L H f L+3 H-1 f L H-4 f L H-1 f L+7 H-11 f L H-1 f L H-4 f L H-13 f L H-7 f L+1

| CI | (coeff)

E (eV)/(nm)

oscillator

assign

λexp (nm)

0.66568(89%) 0.67456(91%) 0.41129(34%) 0.67431(91%) 0.67425(91%) 0.33951(23%) 0.67543(91%) 0.67501(91%) 0.45118(41%) 0.66808(89%) 0.66789(89%) 0.32038(21%) 0.27320(15%) 0.65517(86%) 0.6616(87%) 0.29222(17%) 0.28836(17%)

2.73/455 3.90/318 5.07/244 2.92/425 4.03/308 5.07/244 2.94/421 4.04/307 5.07/244 2.62/473 3.70/335 5.63/220

0.0648 0.1004 0.0435 0.0776 0.1218 0.0380 0.0879 0.1338 0.0528 0.0951 0.1761 0.1163

383

2.40/517 3.54/350 5.39/229

0.0794 0.1435 0.0784

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

IP is relatively similar. These all correspond to removal of an electron from the 5d orbital. As shown in the cation spin densities in Table 4, over 50% of the spin density in all five complexes is on the Re and the remainder is largely shared on group Cl, which basically consists of the analysis of HOMO composition. It is found that the EAs (no matter EA(a) or EA(v)) of 2 and 3 are smaller, whereas for 4 and 5, they are relatively higher than 1, which is in agreement with the trend in LUMO energy. The unpaired spin density is totally on each of the (N∧N) ligands. This is consistent with the LUMO being primarily the glyoxime π* orbital. To value the charge-transfer rate and balance, reorganization energy (λ) was calculated for the studied molecules. According to the Marcus/Hush model,23 the charge (hole or electron) transfer rate k can be expressed by the following formula:

k)

( ) π λkbT

1/

2

(

V2 λ exp p 4kbT

)

where T is the temperature, kb is the Boltzmann constant, λ is the reorganization energy, and V is the coupling matrix element between the cation and molecules, which is dictated by the

354 364

overlap of orbitals. Obviously, the reorganization energy in the charge-transfer process is very important. As shown in Table 4, the λelectron values of 2 are smaller than the λhole values, which suggests that the electron-transfer rate is better than the holetransfer rate. Also, the difference between λhole and λelectron of 2 (0.02) is smaller than that of 4 and 5, which is small and which can greatly improve the charge-transfer balance thus further enhancing the device performance of organic light emitting devices (OLEDs). It is a key point toward the development of novel materials of OLEDs. 3.4. The Absorption Spectra. 3.4.1. Excitation Energies. The calculated absorption energies associated with their oscillator strengths, the main configurations, and their assignments as well as the experimental results of 1-3 are given in Table 5. Figure 2 displays the energy levels of the molecular orbital involved in the electronic transitions of 1-5, which can intuitively understand the transition process, and the lowest-lying absorption transition diagrams of 1-5 are shown in Figure 3. Figures 2 and 3 show that the lowest-lying distinguishable singlet f singlet absorption band of 1 is at 2.73 eV (455 nm) and that the excitation configuration of HOMO-1 f LUMO is responsible for the transition. Table 2 shows that HOMO-1 of 1 is composed of 21.2% dxy(Re), 6.7% π(CO), and 46.1% p(Cl),

Figure 2. Diagrams of the molecular orbital related to the absorptions of 1-5.

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Figure 4. The simulated absorption spectra of complexes 1-3 in methanol solution.

Figure 3. Transitions responsible for the lowest-lying absorption at 455, 425, 421, 473, and 517 nm for 1-5, respectively.

whereas the LUMO is dominantly localized on π* (DHG) with 78.3%. Thus, the lowest-lying absorption at 455 nm for 1 can be mainly assigned to {[dxy(Re) + p(Cl)] f [π* (DHG)]} transition with mixing MLCT/LLCT (ligand-to-ligand charge transfer) transition character. The lowest-lying absorption of 1 at 383 nm is also attributed to MLCT, which basically agrees with our calculated results. The absorptions for 2-5 at 425 nm (2.92 eV), 421 nm (2.94 eV), 473 nm (2.62 eV), and 517 nm (2.40 eV), respectively, have a similar transition character of 1 except for a different d(Re) component. Figure 2 reveals that the lowest-energy absorptions of 2-5 can be attributed to {[dxz(Re) + p(Cl)] f [π* (N∧N)]} transition with the MLCT/LLCT transition character. The second distinguishable absorption bands at 318, 308, 307, 335, and 350 nm dominate these low-energy absorption bands for 1-5, respectively. As shown in Table 5, the transition of H-4 f L can be described as with the mixed character of MLCT, LLCT, and ILCT (intraligand charge transfer). The highest-lying absorptions of 1, 2, and 3 are nearly at 244 nm, and the absorption bands of 4 and 5 appear at 220 and 229 nm, respectively. The absorptions of 1-4 are assigned to the mixed charge transfer transition of MLCT/LLCT/ILCT, but the absorptions of 5 are mainly described as ILCT. 3.4.2. Substituent Effect on the Absorption Spectrum. By comparing the absorptions of 2-5 with 1, it is indicated that the variation of the substituent on the glyoxime changes the excitation energy and spectral property to a different extent. The electron-donating groups result in the increase of excitation energy, and their absorption spectra which arise from the HOMO-1 f LUMO (MLCT/LLCT) show blue shifts as follows (Figure 4): 1 (455 nm) > 2 (425 nm) > 3 (421 nm). Just the opposite, electron-withdrawing groups lead to the decrease of excitation energy, and the spectra are red-shifted (Figure 5), that is, 5 (517 nm) > 4 (473 nm) > 1 (455 nm). The same conclusion has been obtained in a series of halide Re(I) bipyridine complexes19 in which it addressed that the electronreleasing group -CH3 leads to both absorption and emission spectra blue shifts because of the increased energy of LUMO which results in the increased HOMO-LUMO band gap, and electron-withdrawing groups -Br and -COOCH3 lead to a red shift because of the decreased energies of LUMOs which decrease the energy band gaps. The shift trend of the lower energy absorption bands is similar to the lowest-lying absorptions. Additionally, the MLCT

Figure 5. The simulated absorption spectra of complexes 1, 4, and 5 in methanol solution.

Figure 6. Normalized absorption spectra for complex 1 in different solvents.

TABLE 6: The Lowest-Lying Absorptions of 1-5 in Different Solutions solvent

CH3OH

CH3COCH3

CHCl3

C6H5CH3

C6H12

polarity λ(1)/nm λ(2)/nm λ(3)/nm λ(4)/nm λ(5)/nm

6.6 454 425 421 473 516

5.4 458 429 426 479 522

4.4 484 448 446 501 543

2.4 514 472 472 530 570

0.1 523 480 479 538 577

component is more remarkable in the low-energy region of absorptions, and the absorption intensities of 2-5 are stronger than 1. Thus, the participation of metal and substituents in these complexes probably makes the transition occur and enhances the luminescence quantum yields. 3.4.3. SolWent Effect on the Absorption Spectrum. Different solvents can cause different excitation energy and absorption spectrum because of the polarity. In general trends, the absorption of mononuclear compounds are more affected with solvent polarity than binuclear complexes.24 So, it is necessary to discuss the absorption change of 1-5 caused by the solvent polarity.

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TABLE 7: Phosphorescent Emissions of 1-5 in Methanol Solution under the TD-DFT Calculations 1 2 3 4 5

excitation

Ecal (eV)

λcal (nm)

character

L f H-1 L f H-1 L f H-1 L f H-1 L f H-3 L f H-2

2.04 1.88 2.21 1.76 1.61

608 659 562 706 780

[π* (DHG)] f [d(Re) + p(Cl))] (3MLCT/3LLCT) [π* (DMG)] f [d(Re) + p(Cl)] (3MLCT/3LLCT) [π* (CHDG)] f [d(Re) + p(Cl)] (3MLCT/3LLCT) [π* (DBG)] f [d(Re) + p(Cl)] (3MLCT/3LLCT) [π* (DMFG)] f [d(Re) + p(Cl) + π(DMFG)] (3MLCT/3LLCT/3ILCT) [π* (DMFG)] f [d(Re) + p(Cl) + π(CO)] (3MLCT/3LLCT)

TABLE 8: Calculated TD-DFT Excitation Energy (E) and Wavelength (λ) for the Lowest Triplet State of 1 in Different Solutions on the Basis of the Optimized T1 Geometry polarity E (eV) λ (nm)

CH3OH

CH3CN

CH3COCH3

CCl3

C6 H6

CCl4

C6H12

6.6 2.04 608

6.2 2.03 609

5.4 2.01 615

4.4 1.91 650

3.0 1.77 702

1.6 1.76 703

0.1 1.74 713

Figure 6 illustrates the different solvent effect on the absorption spectra of 1 intuitively, where the absorptions are evaluated with PCM method in methanol, acetone, chloroform, benzene, toluene, and cyclohexane. A red shift is observed in the lowest energy electronic transition with decreases in the polarity, which agrees with the experimental conclusion. The same change trend of 2-5 is observed in these solvents. The solvent effect can be explained by the multiparametric method of Kamlet and Taft25 in which the absorption and emission energies are correlated with different solvent properties according to following equation, which is one of the most extensively applied:24

ν¯ ) ν¯ 0 + aR + bβ + p(π* + dδ) where νj0 is the value of the absorption or emission energies in a reference solvent, R is an index of the solvent’s ability to act as a hydrogen-bond donor toward a solute, β is a measure of the ability of a bulk solvent to act as a hydrogen-bond acceptor, π* is an index of the solvent polarity, and δ is polarizability correction. The parameters a, b, p, and d can be retrieved through a multiparametric fitting on various solvents. The presence of electron-donating or electron-withdrawing groups (2-5) in the glyoxime ligand increases the solvent sensitivity of the transition. The same rule is discovered in different solvents (Table 6). The blue shifts are caused by the electron-donating groups, and the red shifts are caused by the electron-withdrawing groups. 3.5. The Lowest-Lying Triplet Excited State. The lowest triplet states T1 of the five complexes have been optimized by the CIS method, and selected geometrical parameters of 1 are depicted in Table 1, while the geometrical parameters of other complexes are shown in Table S4 of the Supporting Information. The calculated results reveal that geometrical parameters of 1 have a small difference from the ground-state structure, and the variation trend of 2-5 is similar with 1. All the Re-C and Re-N bond lengths are relatively longer, and almost all the bond angles are slightly reduced. This is attributed to the minor changes that result from the excitation as well as electrons that transfer from HOMO-1, which is mainly contributed by d(Re) and p(Cl), to LUMO that is largely composed of glyoxime ligand. It also causes the bond lengths of N(1)-C(4) and N(2)-C(5) to lengthen, while the C(4)-C(5) bond shortens. 3.6. The Emission Spectra. The emission spectra of 1-5 are calculated in methanol solvent as listed in Table 7 associated with the emissive energies and the transition character. To conveniently discuss the transition character of emission, we present the partial compositions of frontier molecular orbitals

related to the emissions in Tables S5-S9 of the Supporting Information. In addition, the emission spectra of 1 have also been recorded in various solvents as shown in Table 8. As shown as Table 7, the lowest-energy emissions at 608, 659, 562, and 706 nm of 1-4 are mainly from the transitions of LUMO f HOMO-1. For 1, the LUMO is a π* orbital localized on the glyoxime moiety, while the 52.9% d(Re) and 24.1% p(Cl) contribute to the composition of HOMO-1 mainly (Table S5 of the Supporting Information). Thus, the emission has the [π* (DHG)] f [d(Re) + p(Cl)] (3MLCT/3LLCT) transition character. The lowest-lying absorptions discussed above also arise from the MLCT/LLCT transition. Since the lowest-energy emissions and absorptions have the same symmetry and transition character for the four complexes, the phosphorescent emissions should be the reverse process of the lowest-energy absorptions.26 The effects of the solvents on the spectrum of 1 were evaluated using PCM method in methanol, acetonitrile, acetone, chloroform, benzene, tetrachloride, and cyclohexane. The calculated emission spectra have a red shift with a decrease of polarity from methanol to cyclohexane (Table 8). This change trend is consistent with the complex [Re(4,4′-(COOEt)2-2,2′bpy)(CO)3py]PF6 (bpy ) bipyridine, py ) pyridine),27 while in opposition to the complexes [Re(R2bpy)(CO)3X] (R ) H, t-Bu; X ) Cl-, OTf-, C≡CpyRe(R2bpy)(CO)3), the multiparametric fitting of Kamlet and Taft25 has also been applied to explain the change rule.23 The change rule will provide useful guidance for future experiments. 4. Conclusions The present work investigated the ground- and excited-state geometries, absorption, and phosphorescence properties of five Re(I) complexes by DFT, CIS, and TDDFT methods. The calculated ionization potentials (IP), electron affinities (EA), and orbital energy indicate that the energies of LUMOs increase obviously with the introduction of the electron-donating groups, and the change results in increased HOMO-LUMO energy gaps. Therefore, the absorptions exhibit blue shifts in the order 2 > 3. The reorganization energy (λ) calculations indicate that with the incorporation of electron-donating groups into one molecule (2), the device performance will be improved because of the more balanceable charge-transfer abilities. In addition, the solvent effect on the spectra is evaluated using PCM method in a wide range of solvents with different polarity. It is discovered that both the lowest-lying absorptions and emissions are red-shifted with the decrease of solvent polarity, and the shift becomes further with the smaller polarity. These theoretical studies can provide constructive information in discovering new efficient phosphorescent materials.

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