Effect of Substituents on Electronic Structure and Photophysical Properties of Re(I)(CO)3Cl(R-2, 2’-Bipyridine) Complex: DFT/TDDFT Study

The electronic structure, absorption and emission spectra, as well as phosphorescence efficiency of Re(I) tricarbonyl complexes of a general formula fac-[Re(I)(CO)3(L)(R-N^N)](L = Cl; N^N = 2, 2’-bipyridine; R = -H, 1; -NO2, 2; -PhNO2, 3; -NH2, 4; -TPA (triphenylamine), 5) were investigated by using density functional theory(DFT) and time dependents density functional theory (TDDFT) methods. The calculated results reveal that introductions of the Electron with drawing group (EWG) and Electron donating group (EDG) on the R position of 2, 2’-bipyridine ligand. When EWG (-NO2 and -PhNO2) are introduced into complex 2 and 3, the lowest energy absorption and emission bands are red shifted compared with that of complex 1. On the contrary, the introduction of the EDG (-NH2 and -TPA) in complex 4 and 5 cause corresponding blue shifted. The solvent effect on absorption and emission spectrum indicates that the lowest energy absorption and emission bands have red shifts with the decrease of solvent polarity. The electronic affinity (EA), ionization potential (IP) and reorganization energy (λ) results show that complex 5 is suitable to be used as an emitter in phosphorescence organic light emitting diodes (PHOLEDs). Meanwhile the emission quantum yield of complex 5 is possibly higher than that of other complexes.


Introduction
Organic light emitting diodes (OLEDs) are solid state electronic devices which emit light in response to the applied electric current. Among different types of OLEDs, PHOLEDs are the classes of OLED that use the principle of phosphorescence to offer four times the internal efficiency of conventional fluorescent OLEDs. Light emitting organic materials has phosphorescent materials which serve as recombination centers for electrons and holes to produce the electronic excited states are mainly captivating. Because, they harvest light from both singlet and triplet excitons and enabling the devices with close to 100% internal quantum efficiency [1]. However, exploration of novel materials including emitting materials and host materials is crucial to push the performance of the PHOLEDs [2]. PHOLEDs have attracted worldwide attention over the past decades due to their unique features and huge potential for flat panel, flexible display, energy saving solid state lighting and virtual reality [3].
The highly efficient PHOLEDs can be designed from the 3 rd row d 6 transition metal complexes with appropriate organic ligand because, existence of transition metal in complex exhibit strong spin orbit coupling (SOC) which substantially accelerates singlet to triplet intersystem crossing ISC [4][5][6]. Selection of organic ligand allows different electronic transitions occurring between different energy states associated with metal atom is important to design highly efficient luminescent complexes [7][8][9]. The specific photophysical properties of bidentate hetero-aromatic diamine ligand complexes with third row d 6 transition metal ions such as Re(I), Ru(II) and Os(II) are significant. Among them Re(I) tricarbonyl complexes with substituted 2, 2'bipyridine ligand attached rhenium often show intense and long lived luminescence. 2, 2'-bipyridine is a bidentate ligand with powerful binding abilities towards Re(I) metal ion. It can be easily modified by the introduction of different substituent groups at its different positions. Introduction of electron with drawing group (EWG) or electron withdrawing group (EDG) substituent is useful to tune the energy level of 2, 2'-bipyridine ligand to provide high efficiency PHOLED [10][11][12][13][14][15].
On the other hand, in the PHOLED system, unbalanced charge transport for electrons and holes in the emission layer results in low luminescence efficiency and less internal quantum efficiency. Because, these complexes are fairly good electron transport materials while hole transport ability is poorer [16]. The authors has tried to solve the problem of the internal quantum efficiency and low luminescence efficiency of this complex in the PHOLED system by tuning electronic structure and photophysical properties of Re(I) tricarbonyl complexes with varying substituent on the 2, 2'-bipyridine ligand theoretically through DFT and TDDFT study. To overcome less internal quantum efficiency and low luminescence efficiency PHOLEDs problem, incorporation of hole transporting groups to the 2, 2'-bipyridine ligand of Re(I) tricarbonyl complex can be an alternative option. The structures of fac-[Re(I)(CO) 3 L(R-N^N)] where, (L = Cl, N^N = 2, 2'-bipyridine, R = -H, 1; -NO 2 , 2; -PhNO 2 , 3; -NH 2 , 4; -TPA, 5 under the study shown as below:

Computational Methodology
The singlet ground state (S 0 ) and lowest lying triplet excited state (T 1 ) geometries of all complexes were optimized using the DFT method in the gas phase [2]. The B3LYP exchange correlation functional provides enough accurate information on the ground and excited state properties of the complex [17][18]. A double-ζ quality LANL2DZ basis set is applied for Re atom together with 6-311++G(d, p) basis set for C, H, N, O and Cl atoms. Both LANL2DZ for Re and 6-311++G(d, p) for other atoms basis set are also written in the additional input under Gaussian calculation set up [19][20][21][22][23]. Vibrational frequency analyses were performed to confirm that the optimized structures were true minimum or stable structures. Vibrational frequency calculations in which there is an absence of imaginary frequency for all configurations, confirm that they are minimum point on the potential energy surface. HOMO and LUMO contour plot and energy level of all complexes were obtained from optimized structures.
Excited state calculations were studied using the TDDFT method on the basis of an optimized structure of all the complexes with the same functional and basis sets [24][25]. On the basis of optimized S 0 and T 1 structures, TDDFT method was used to calculate the electronic absorption and phosphorescence spectra of the complexes, respectively. Absorption spectra of 1 -5 complexes were simulated in the GAMESS software to obtain best spectra. In order to incorporate the effect of the solvent around the molecule, the polarizable continuum model (PCM) is used in the TDDFT calculation [5]. The calculated electronic density plots for the FMOs were performed by Gauss View 5.0.8 software. In addition, the positive and negative ions with regard to ''electron -hole'' creation are relevant to their use as OLED materials. Thus, the ionization potentials (IP), electron affinities (EA) and reorganization energies (λ) were obtained by comparing the energy levels of the neutral molecule with the positive ions and negative ions, respectively. All calculations were performed using Gaussian 09 software package [26].

The Optimized Geometries in the Ground States and Lowest Triplet States
The structures of the studied Re(I) tricarbonyl with substituted R positions of 2, 2'-bipyridine ligand by -H, -NO 2 , -PhNO 2 , -NH 2 and -TPA complexes are shown in Figure 1. Optimized ground state geometries of 1 -5 complexes are shown in Figure 2. The selected bond lengths and bond angles of 1 -5 complexes in the optimized ground state (S 0 ) and the lowest lying triplet state (T 1 ), as well as the experimental values of complex 1 based on crystallographic data taken from the literature [34] are collected in Table 1. The geometry consists of the 2, 2'-bipyridine ligand, three carbonyl ligands and chlorine atom around the Rhenium atom. The stability of optimized geometries of all complexes was confirmed by frequency calculations in which there is no imaginary frequency for all configurations. It can be seen from Figures 1 and 2 that Re(I) tricarbonyl with substituted R position of 2, 2'-bipyridine ligand complexes have an analogous octahedral coordination around the Re atom with Cl atom, 2, 2'-bipyridine and three CO ligands in a facial configuration. Table 1 shows that the bond lengths and bond angles in 1-5 complexes display typical features of Re(I) tricarbonyl diamine complexes. The optimized parameters of complex 1 are in good agreement with the experimental values based upon crystallographic data taken from literature [34]. It gives positive indication on the reliability of the theoretical method. The slight deviations between calculated and experimental values are due to the fact that the effects of crystal packing and chemical environment. The effects of both crystal packing and chemical environment are not considered in theoretical calculations. Theoretical calculations refer to gas phase but experimental results are from the close packed crystal lattice. The substitution of the R position of 2, 2'-bipyridine ligand by different EWG and EDG give rise to negligible changes for the bond lengths and bond angles in 1 -5 complexes in S 0 state as it is shown in Table 1. There is a typical angle about 90 0 among three CO ligands in the fac-Re(CO) 3 + unit for all studied complexes. And the axial Re-C bond distance is shorter than the equatorial Re-C bond distances in each complex. This is attributed to different ligand to metal back bonding ability which is that the axial CO opposite to Cl atom. Therefore, the introduction of R = -H, 1; -NO 2 , 2; -PhNO 2 , 3; -NH 2 , 4 and -TPA, 5 groups on the R position of 2, 2'bipyridine ligand takes a minor effect on their geometric structures in S 0 state.
The calculated geometrical parameters of 1 -5 complexes in the triplet state (T 1 ) are also given in Table 1. It also reveals that the introduction of different substituent groups on the R position of 2, 2'-bipyridine ligand gives rise to a little effect on their geometric structures in T 1 state. However, the bond lengths and bond angles of 1 -5 complexes in T 1 state have obvious changes compared with those in S 0 state. Specifically, the bond lengths of Re-N, Re-Cl are contracted while the bond lengths of Re-C was relaxed. This shows that the interaction between Re(I) and 2, 2'-bipyridine ligand is strengthened, while the interactions between Re(I) and three CO ligands are weakened in T 1 state. Therefore, 2, 2'bipyridine ligand have a greater effect on FMOs of these complexes in T 1 state. The different strengths between Re(I) and 2, 2'-bipyridine ligand or CO ligands will lead to different electron transition characters. Experimental values from [34]. Calculated optimized parameters shows octahedral coordination structure and interaction between Re (I) and 2, 2'-bipyridine ligand of all complexes.

Molecular Orbital Properties
The properties of the frontal molecular orbital (FMOs) have a significant effect on the excited states and electronic transitions of OLED materials. It is well known that the optical properties of the complexes are associated with FMOs especially, HOMO and LUMO [27].
The contour plots of HOMO (H) and LUMO (L) of 1 -5 complexes are shown in Figure 3 and the main FMO energy levels are plotted in Figure 4.

Absorption Spectra
Based on the optimized ground state geometry, the absorption properties of 1 -5 complexes have been determined. TDDFT/B3LYP/LANL2DZ (for Re atom) and 6-311++G(d, p) (for C, H, N, O and Cl atoms) theoretical method with PCM in CH 2 Cl 2 media was used to calculate absorption spectra of all complexes. The transition behavior, the relevant energies/wavelengths, oscillator strength, dominant orbital excitations with configuration interaction (CI) coefficients and their assignments as well as experimental values from the literature [34] of complex 1 are collected in Table 2. By using GAMESS software, the corresponding simulated UV-Visible absorption spectra of 1 -5 complexes are sketched in Figure 5. For complex 1, the calculated lowest lying energy absorption is at 423 nm, which is assigned to electronic transition from H-1 to L. HOMO is contributed by [d(Re) + p(Cl) + π(CO)], while LUMO predominantly located on π*(2, 2'-bipyridine). H-1 and L+1 have a similar charge transfer character with H and L. The MLCT type is commonly observed in the lowest energy transitions for transition metal complex [28][29].
So this electronic transition can be described as [d(Re) + p(Cl) + π(CO)] → π* (2, 2'-bipyridine) with MLCT/XLCT/LLCT is in good agreement with the experimental value 420 nm. The moderately intense band is at 448 nm. Similarly, the transition of this band is from H to L with characteristic MLCT/XLCT/LLCT. The strong absorption at 316 nm is from H to L+2 with LLCT/XLCT character. Therefore, it indicates that the calculated value is precise and reliable to explain absorption spectra. As observed in table 2, the lowest energy and moderately intense absorption bands of complexes 2 -5 have a similar transition character as compared to the absorption bands of complex 1. But the strong absorption bands of complexes 2 and 3 are different from that of complex 1. Specifically, for complex 2, the strongest absorption band at 420 nm is from H-3 to L with LLCT/XLCT character. For complex 3, the strong absorption band at 419 nm is also from H-1 to L+1 with MLCT/LLCT/XLCT character. Moreover, for complexes 4 and 5, the strong absorption bands are different from bands of complex 1.
For complex 4, the transition character of a strong absorption band is H-3 to L which is having a mixed character of XLCT/ILCT while the absorption band of complex 5 is assigned to H-3 to L and H to L+2 excitations with mixing MLCT/XLCT/LLCT. From Figure 5, it can intuitively be seen that the calculated absorption bands for 1 -5 complexes have almost identical profiles but the positions of absorption bands are different. It is noteworthy that the contribution of MLCT is more remarkable in the lowest energy absorption bands with the large compositions of Re atom in HOMOs and HOMO-1s. The lower energy absorption bands of 1 -5 complexes follow the order: 2 (634nm) > 3 (505nm) > 1 (423nm) > 4 (404nm) > 5 (400nm). It reveals that the introduction of EWG on 2, 2'bipyridine ligand (mainly for complex 2 and 3) absorption bands red shifted (longer wavelength). On the hand when EDG substituents introduced on 2, 2'-bipyridine ligand (complex 4 and 5) the absorption band become the blue shifted (shorter wavelength).   The contribution of MLCT/XLCT/LLCT is remarkable in lowest energy absorption band with the order of 2 > 3 > 1 > 4 > 5. All spectra have identical profiles but they have different absorption bands.

Phosphorescence Spectra
With the optimized T 1 structures as the starting point, the emission spectra of complexes 1 -5 were determined by the TDDFT/B3LYP methods with PCM in CH 2 Cl 2 media. The energy/wavelengths, dominant transitions with larger CI coefficients and their assignments are collected in Table 3. The contour plots of HOMO and LUMO for all complexes are shown in Figure 6 and main FMOs energy levels are plotted in Figure 7. Table 3 shows that the energy emissions of 1 -5 complexes mainly originate from LUMO to HOMO transition. According to Table 3, it also can be seen that LUMOs are mainly localized on π*(2, 2'-bipyridine) orbital for all complexes, while HOMOs are primarily on [d(Re)], [p(Cl)] and π(CO) ligands. To intuitively understand the emission transition, the plots of HOMO and LUMO emissions of 1 -5 complexes are presented in Figure 6.
For 1 -5 complexes, HOMO energy levels are similar to each other, but LUMO energy levels present an obvious variation. The introduction of -NO 2 and PhNO 2 on R position of 2, 2'-bipyridine ligand can decrease the E LUMO . The introduction of -NH 2 and -TPA increases the E LUMO . So, the introductions of -NO 2 and -PhNO 2 lead to the lowest energy emission bands red shifted relative to complex 1. The introduction of -NH 2 group in complex 4 and -TPA in complex 5 can cause a corresponding blue shift. Moreover, complex 5 emit light in the visible range, but other complexes occur in the near-infrared region. There is no experimental assignment for the emissions of these complexes [30]. Therefore, the introduction of stronger electron donating group on the R positions of 2, 2'-bipyridine ligand blue-shifted the spectra with the lowest energy emission band.

Effect of Solvent on Absorption and Emission Spectra
Different solvents have different polarity. Different solvents can cause different excitation energies due to their polarity. For the lowest energy absorption and emission wavelengths, solvent effects are assessed using the PCM method as shown in Table 4 for complexes 1 -5. Both the lowest energy absorption and emission bands have red shifted (longer wavelength) with the decrease in solvent polarity while blue shifted (shorter wavelength) with the increase in solvent polarity for all complexes. Compared with the experimental method, it is easy for theoretical calculations to change solvents. This is another advantage of theoretical calculations.

Electronic Affinity (EA), Ionization Potential (IP) and Reorganization Energy (λ)
The electronic affinity (EA), ionization potential (IP) and reorganization energy (λ) are very significant in performance ability of PHOLED materials. IP and EA are usually used to assess the energy barrier for the injection of holes and electrons from the anode into the emitting materials [31]. The EA can be either for vertical excitations or adiabatic excitations denoted by EA (v) and EA (a). Similarly, IP is denoted by IP (v) and IP (a). The IP V is defined as the energy difference between the cation and its neutral molecule at the equilibrium geometry of the neutral molecule. The IP a is defined as the energy difference between the cation and its neutral molecule at their own equilibrium geometries. The E Av is defined as the energy released when the electron is added to the neutral molecule computed as the energy difference between the neutral form and the anion. The EA a is defined as the energy difference between the neutral molecule and its anion at their own equilibrium geometries. The HEP is the energy difference between neutral molecules and cationic systems calculated using a cationic geometric structure. Similarly, the EEP is the energy difference between neutral molecules and anionic systems calculated using anionic geometric structures. In this study vertical and adiabatic of EA and IP (EAv, EAa, IPv, and IPa), EEP, HEP, electron transport reorganization energy (λ electron ), hole transport reorganization energy (λ hole ) and the difference between λ hole and λ electron for all complexes were calculated with DFT method and collected in Table 5. The introduction of EWG on R position facilitates the electron injection ability, while the introduction of EDG facilitates the hole injection ability. As shown in Table 5, it can be found that all of the complexes possess lower λ electron in comparison to λ hole except complex 5, which reveal that electron transporting performance is slightly better than hole transporting performance. This means that all of these complexes can be used as electron transporting materials. But, as it can be seen complex 5 has a relatively small difference between λ electron and λ hole which can improve the charge transfer balance and further enhance the device performance of PHOLEDs. So complex 5 is more suitable to be used as an emitter in OLEDs in this regard. The above analysis result reveals that the different substituent groups on the R position of 2, 2'bipyridine ligand can influence charge transfer rates of electrons and holes. In this case, we hope that theoretical investigations can guide experiment to develop novel and efficient materials of OLEDs.

The Emission Quantum Yield in CH 2 Cl 2 Media
The emission quantum yield (Φ) can be affected by the competition between radiative decay rate constant (K r ) and non-radiative decay rate constant (K nr ) [32]. r r em r nr Where, τ em is the emission decay time. From the above formula, the large K r and small K nr are required to increase the value of emission quantum yield (Φ). The K r and K nr can expressed as: 2 S1 2 S1 S0 T1 r 2 S1-T1 Where, α and β are constant, µ S1 is transition electric dipole moment from S 0 to S 1 . ∆E S1-T1 is the energy gap between S 1 and T 1 states, E T1 is the energy of the lowest triplet excited state for phosphorescence and n, h and are refractive index, plank's constant and permittivity in a vacuum, respectively. Therefore, the variation of Φ can be qualitatively calculated from the above formulas. According to above equation when E T1 value increase, K r may increase and K nr may decrease. The corresponding data are summarized in Table 6. It can be seen from the table that complex 5 has the largest E T1 (1.581 eV), which may increase the value of Φ. The SOC effects can be elucidated mainly from the energy gap between S 1 and T 1 state (∆E S1-T1 ) [33]. The S 1 and T 1 ISC plays a significant role in phosphorescent process. The ISC rate decreases exponentially as ∆E S1-T1 increases. The minimum ∆E S1-T1 will enhance ISC rate and transition moment which could increase K r . Table 6 shows that complex 5 has the largest E T1 (1.581 eV), the smallest value of ∆E S1-T1 (1.174 eV), and largest µ S1 (6.3D) among the complexes, which may make it has a larger emission quantum yield (Φ) than other complexes. So the designed complex 5 is possible to be good candidate for phosphorescent materials among the studied complexes.

Conclusion
In this study, the five Re(I) tricarbonyl complexes containing 2, 2'-bipyridine ligand with different substituted groups (-H, -NO 2 , -PhNO 2 , NH 2 and -TPA) have been investigated by DFT/TDDFT methods. Specifically, the S 0 and T 1 state geometries, FMOs, absorption and phosphorescence spectra, solvent effect on absorption and emission spectra, electronic affinity, ionization potential, reorganization energy and emission quantum yield of all complexes were studied. The calculated results suggest that, the introductions of different groups on the R position of 2, 2'-bipyridine ligand have significant effect on the electronic structures and photophysical properties such as absorption and emission spectra, charge injection and transfer abilities and emission quantum yield. On the basis of FMOs analysis, for complexes 1 -5, HOMOs energy levels have small changes, but LUMOs energy levels reveal obvious variation. Specifically, introductions of EWGs (-NO 2 and -PhNO 2 ) can decrease the energy level of LUMO resulting in narrower energy gap, which lead to red shifted of the lowest energy absorption bands in comparison with that of complex 1. On the contrary, introductions of EDG (-NH 2 and -TPA) can make the energy level of LUMO increasing, which cause the corresponding blue-shifted. The solvent effect on absorption and emission spectra indicates that lowest-energy absorption and emission bands have red-shift with the decrease of solvent polarity for these complexes. From EA, IP and λ results, we also can conclude that these complexes can be used as electron transporting materials. Moreover, the difference between λ electron and λ hole of complex 5 is the smallest among these complexes, enhancing the device performance of OLEDs further. The emission quantum yield of complex 5 is possibly higher than other complexes. Therefore, complex 5 is more suitable to be used as an emitter in the PHOLEDs. Lastly, theoretical investigations can provide constructive information in the design and synthesis of novel and high-efficiency OLEDs materials. The Re(I) tricarbonyl complexes with incorporated hole transporting group to the 2, 2'-bipyridine ligand exhibit wonderful luminescent properties.