Optical Excitation in DonorâPtâAcceptor Complexes: Role of the

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Optical Excitation in Donor-Pt-Acceptor Complexes: Role of the Structure Zu-Yong Gong, Sai Duan, Guangjun Tian, Guozhen Zhang, Jun Jiang, and Yi Luo J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b03072 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 8, 2016

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The Journal of Physical Chemistry

Optical Excitation in Donor-Pt-Acceptor Complexes: Role of the Structure Zu-Yong Gong,†,‡,§ Sai Duan,‡,§ Guangjun Tian,‡,¶ Guozhen Zhang,† Jun Jiang,∗,† and Yi Luo†,‡ †Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, 230026, Anhui. P. R. China ‡Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden ¶College of Science, Yanshan University, Qinhuangdao 066004, China §Contributed equally to this work E-mail: [email protected] Phone: +86 (551) 63600029

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Abstract The optical properties of the Pt complexes in the form of donor-metal-acceptor (D-M-A) were studied at the first principles level. Calculated results show that, for the frontier molecular orbitals (MOs) of a D-M-A structure, the energies of unoccupied frontier MO can be mainly determined by the interaction between M and A. Whereas, the M-A and M-D interactions both determine the energies of occupied frontier MO. By developing a straightforward transition dipole decomposition method, we found that not only the local excitations in D but also those in A can significantly contribute to the charge-transfer (CT) excitation. Furthermore, the calculations also demonstrate that, by tuning the dihedral angle between D and A, the transition energy and probability can be precisely controlled so as to broaden the spectrum region of photo-absorption. For the D-M-A molecule with a delocalized π system in A, the CT excitation barely affects the electronic structures of metal, suggesting that the oxidation state of the metal can be kept during the excitation. These understanding for the optical properties of the DM-A molecule would be useful for the design of dye-sensitized solar cells, photocatalysis and luminescence systems.

Introduction Optical properties of metallic complexes are very important in many fields. 1–5 For example, the performance of many dye-sensitized solar cells (DSSCs) relies on the abilities of their metallic complex structures to convert photon energy into electronic energy through chargetransfer (CT) excitation. Considering the promising potential of DSSCs in solar energy applications, 6 it is thus important to get in-depth understanding of the optical excitation process in the metallic complex. Moreover, another important field of photo-luminescence makes a great use of the excitation states of metallic complexes. 7 Four coordinated platinum (Pt) complexes are a typical type of dye with promising potential in applications, mainly due to some unique advantages. 8 The most important 2


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advantage, from the theoretical point of view, is that Pt complexes can be easily “modular” synthesized with different ligands and thereby provide tunable and clear electron transfer routes . 8–10 Therefore, the Pt complex can serve as an ideal model system to investigate the optical properties of metallic complexes. The theory for the CT state can be traced back to the pioneering works of Mulliken 11 and Murrell, 12 who decomposed the whole system into a donor (D) and an acceptor (A). In such a framework, they found that, the transition energy of the CT states, can be described as ID − EA − C, where ID is the ionization potential of the donor, EA is the electron affinity of the acceptor, and C is the electrostatic interaction between D+ and A – in the CT states. 12,13 For the transition dipole moment, which holds the key to an electron transition from the initial state to final state, is determined by the necessary overlap between the wavefunctions of A and D, as well as the selection rules due to the symmetry requirement based on group theory. 11,12 It was also noted that the transition of the CT states can “borrow” intensity from locally excited D because in CT states the molecular orbitals (MOs) of fragment A – can be easily hybridized by those of D. 12 An alternative way to consider the local excitation in the CT states is the “molecules in molecules” method, which expands electronic states by ground configuration, locally excited stats, and CT states. 14 Tretiak and co-workers have developed a visualization method at the combination level of a semiempirical model Hamiltonian and the time dependent methods based on the decomposition of transition density matrix for characterization of the excited states in (D−π−A)n systems. 15 This method has been extended to the first principles levels by Sun et al. 16,17 Recently, Hasegawa et al. proposed a localized molecular orbitals based method to analysis the excited wave function. 18 Nevertheless, the electronic wavefunction hybridization between A and D, together with the complicated interactions among the molecular and metallic parts, have brought big complexity to the analysis of various contributions to the CT process. The lack of a convenient analysis tool, especially for transition dipole moments, has limited the design of molecular groups to control or optimize the photo-electron conversion performance.

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In this study, we focus on the optical properties associated with the CT states of Pt complexes. Using the Pt complexes as examples, we can extend the theoretical investigation from D-A system to D-M-A system. We found that, the interactions between D-M and A-M play an important role to the excited energies of D-M-A. For the CT transition, we have developed a convenient transition dipole decomposition method to assign the CT excitation contributions to different group pairs. It helps us to reveal that, for the D-M-A system with small conjugated π system, the “borrowing” contribution in the transition dipole moment is also significant for the locally excited A. These information then enabled to us to achieve precise control of transition dipole moment by tuning the dihedral angle between D and A, so as to broaden the spectrum region of the D-M-A system in absorbing photons. This work would thus lead to a novel design strategy of metal complex toward improved photo-electron conversion ability.

Computational Details The current study considered five representative D-M-A type Pt complexes as shown in Figure 1. Here, two different acceptors of 4,4′ -di-tert-butyl-2,2′ -bipyridyl (tBu2 bpy) and 4,4′ ,4′′ tri-tert-butyl-2,2′ :6′ ,2′′ -terpyridine (tBu3 tpy), were used in molecules 1, 2 3 and molecules 4, 5, respectively. Five different donors, phenylacetylide, phenylsulfur, 1,2-benzenedithiolate, phenylacetylide, and phenyl-sulfur, were successively adopted in the molecules 1, 19–21 2, 22 3, 23–25 4 26–28 and 5. 29 These ligands are widely used in DSSCs and luminescence spectroscopy due to their unique optical properties. 8,20–27,29 Another purpose of using these ligands is that the complexes can be categorized into the planar structures between A and D π systems (molecules 1, 3, and 4) and the distorted structures (molecules 2 and 5) as shown in Figure 1. All molecular structures at their ground states (S0 ) were optimized by density functional theory (DFT) at B3LYP 30 level with the double zeta basis set 6-31G(d) 31 for H, C, N, and

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Figure 1: Structures of the investigated Pt complexes (molecules 1-5) as optimized at the IEFPCM-B3LYP/6-31G(d) level. Ochre, yellow, blue, cyan, and white balls represent Pt, S, N, C, and H atom, respectively. Cartesian coordinate axes are plotted. S, and core pseudo potential basis set Lanl2dz 32 for metal Pt. Here the Lanl2dz basis set takes the relativistic effects into account. Although conventional DFT functionals could underestimate the excitation energy of the charge-transfer states, especially for the long-range charge-transfer states in weakly interacting molecular complexes, 33 for the strongly bonded Pt complexes, the currently used method is still reliable and has been widely used in the theoretical studies. 34 To consider the solvatochromic CT absorption of the complexes, 8,20,35 solvent effects of acetonitrile were considered by the polarizable continuum model using the integral equation formalism variant (IEFPCM). 36 During the optimization, all degrees of freedom were allowed to relax. Based on the optimized structures, time-dependent density functional theory (TDDFT) 37 calculations with the B3LYP functional were performed to calculate the electronic excited states of these molecules. The non-equilibrium IEFPCM solvation was used for the electron density calculations of the excited states. The transi5



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tion dipole moments between S0 and excited states were calculated, and the corresponding ultraviolet-visible (UV-vis) spectra were generated by a Lorentzian function with a full width at the half-maximum (FWHM) of 0.1 eV. All optimization of electronic and geometrical structures were performed by Gaussian 09 suite of program. 38 Based on the electronic structure, the Mulliken population analysis of MOs was performed. 39 The group contribution for MO can be expressed as X

Pi (A) =

c∗iv ciu Svu ,

(1)

v∈A or u∈A

where i is the index of specific MO, A is the group D, A, or M, v and u represent the indices of basis functions, c is the corresponding MO coefficient, S is the overlap matrix. We should emphasize that groups are used as the unit in the analysis, which largely reduces the basis set effect in conventional Mulliken analyses. For the transition dipole moment, the contribution for the excitation from one group (A)to another group (B) can be calculated by X

µi (A → B) =

i Duv rvu ,

(2)

v∈A and u∈B

where i is the specific transition between S0 and the ith excited state, v and u also represent the basis functions, D i is the corresponding transition density matrix calculated from the wavefunctions of S0 , r represent the multipole matrices. Here, to address the so-called “assignment problem” in TDDFT, 40 we used the method proposed by Casida and co-workers for transition density matrix calculations. 41 The advantage of this method is that the total transition dipole moment can be calculated by simply adding µi (A → B) of all individual group pairs, which allows the quantitative analysis for the transition dipole moment. For the electron destruction, the Bader charge analysis 42,43 was performed.

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1

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UV-Vis / a.u.

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3

4

5 200

400

600

800

Wavelength / nm Figure 2: Calculated UV-vis spectra of molecules 1 to 5 (from top to bottom) at the IEFPCM-B3LYP/6-31G(d) level. The dotted lines represent the corresponding experimental spectra of 1, 3 and 4 extracted from Refs. 19, 24, and 28, respectively. All calculated spectra have been shifted by 0.25 eV and broadened by a Lorentzian function with an FWHM of 0.1 eV.

Results and discussions The calculated UV-vis spectra of molecules 1 to 5 in the range of 200 to 800 nm were depicted in Figure 2. The calculated results show that all molecules exhibit a couple of peaks of absorptions in the wavelength range of 250 to 400 nm. These ultraviolet adsorption bands are mainly attributed to the local excitation of the A’s, which are not interested in the current investigation. In the range of wavelength larger than 400 nm, the UV-vis spectra show significant difference among the five molecules. The wavelength of the first significant peak is in the order of 5 > 3 > 2 > 4 > 1, which is in agreement with experimental observations 19,24,28 (see Figure 2 for details). Meanwhile, the order of corresponding absorbance intensity is 1 > 3 > 4 ≫ 2 > 5. For the application in DSSCs, the absorption spectrum region, absorbance intensity, and the amount of transferring electrons are three important 7



parameters for the identification of a good dye. The calculated UV-vis spectra in Figure 2 show that both the absorption energy and the intensity vary in a broad range for the considered model molecules, which indicates that they are all good candidates for systematically exploring the optical properties of the Pt complexes.

-2 LUMO

MO Energy / eV

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-3 -4 -5 HOMO

-6 -7

1

2

3

4

5

Figure 3: The major contribution of orbital transition for the first band of calculated UV-vis spectra for molecules 1 to 5. To investigate the absorption energy, the energies of the frontier MOs and the transitions that has the major contribution to the first absorption band in the UV-vis spectra are depicted in Figure 3. For the molecules 1 and 2, the major contributions to the first significant band are both the transition between S0 and the second excited state (S2 ). These bands correspond to the transition from the second highest occupied molecular orbital (HOMO-1) to the lowest unoccupied molecular orbital (LUMO). For the other molecules, the excitation between S0 and the first excited state (S1 ) dominates the first bands, which relates to the transition between the highest occupied molecular orbital (HOMO) to LUMO. Figure 3 also show that, molecule 5 has the smallest energy gap between two MOs that are involved in the electron transition among all molecules. As a result, molecule 5 has the longest wave8


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length for the first absorption band in Figure 2. It is noted that the energy gap between two MOs in an electron transition is the dominant factor that determines the energy of absorbed photon, as stated by the general transition theory proposed by Mulliken and Murrell. 11,12 These results could be attributed to the intrinsic interactions between metal and ligands. The detailed analysis for the transition MOs shows that, for the considered molecules, the occupied MOs and unoccupied MOs have different behaviors for different ligands. In the previous studies for Ruthenium complexes, the conclusion is that A and D separately affect the unoccupied MOs and the occupied MOs. 44 As shown in Figure 3, the energies of unoccupied MO are similar for molecules with the same A (see the LUMOs of molecules 1 to 3 and those of 4, 5), which is consistent with the previous conclusion. Whereas, the energies of occupied MO could be significantly affected by both A and D (see the HOMOs of 1, 4 and those of 2, 5). It is thus easier to adjust wavelength of absorbed light by tuning D than tuning A. In addition, the energy level of LUMO which is sensitive to the structure of A needs to match the conduction band of semi-conductors in DSSCs. 6,45 Therefore, in such applications, it appears to be more feasible to manipulate the structure of D. As mentioned before, the corresponding transition of the first band is not always related to the transition between the S0 and S1 . Thus the analysis of the frontier MOs is desirable for investigating the transition intensity as well as the effective amount of transferred electrons. Diagrams of all frontier MOs, including HOMO, HOMO-1, and LUMO of molecules 1 to 5, are shown in Figure 4. We should emphasize that, for all molecules, the HOMO to LUMO excitation leads to the first excited state S1 . For molecules 1 to 3, S2 is the result of HOMO-1 to LUMO transition. For the molecule 4, the HOMO-1 to LUMO transition corresponds to the third excited state (S3 ). There is no excited state corresponds to the single HOMO-1 to LUMO transition in the molecule 5. We have also computed the transition dipole moment between the frontier MOs shown in Figure 4. The consistency between two transition dipole moment values within each transition show that these transitions constitute the major part of transition between S0 and the corresponding excited states, in line with the analysis of

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Figure 4: Calculated HOMO, LUMO, and HOMO-1 (from left to right) of molecules 1 to 5 (from top to bottom) at IEFPCM-B3LYP/6-31G(d) level. The absolute value of isosurface for all MOs is 0.025. The values above the arrow are transition dipole moment between the ground state and the corresponding excited state, while the values below the arrows are transition dipole moment between corresponding MOs. MO contributions for all excited states. As shown in Figure 4, the LUMOs of all molecules are similar in terms of their compositions which consist of π orbital of D and dxz orbital of Pt atom. This is consistent 10


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Table 1: Decomposition of the z-component for transition dipole moment between S0 and excited states into M (Metallic Pt), A (Acceptor), and D (Donor). The largest two contributions are showed in the bold font. From To 1a 2a 3b 4b 5b M M 0.16 -0.05 -0.09 0.08 0.02 M A -0.01 0.01 0.00 -0.06 0.06 M D -0.27 0.05 0.08 -0.12 -0.07 A M -0.10 -0.02 -0.09 -0.08 0.09 A A -0.84 -0.50 -0.88 -0.46 0.22 A D 0.02 -0.01 0.01 0.08 -0.05 D M -0.12 0.18 0.24 0.06 -0.21 D A -0.04 -0.02 -0.01 0.04 0.05 D D -0.66 -0.50 -0.91 -1.35 0.66 Tot. -1.87 -0.87 -1.65 -1.82 0.78 a S0 to S2 b S0 to S1 with the energy levels of unoccupied MO (see Figure 3). The cause can originate from the local C2v symmetry if only consider the acceptors and the Pt atom. In the local C2v symmetry, all LUMOs belong to b1 irreducible representation. In contrast, the occupied MOs (HOMO and HOMO-1) are different. If further considering the donors, for the molecules 1, 3, and 4, the local C2v symmetry is maintained. The irreducible representations for HOMO of the molecules 1, 3, and 4 are a2 , b1 , and b1 , respectively. Meanwhile, for HOMO-1 of the molecules 1, 3, and 4, the irreducible representations are b1 , a2 , and b2 , respectively. According to the point group theory, 46 in C2v symmetry, the transition between a2 and b1 only has y-component, transition between b1 and b1 only has z-component, and transition between b2 and b1 is forbidden. The calculated transition dipole moments agree with this prediction. For the molecules 2 and 5, when considering the donors, the local C2v symmetry is not kept and C1 symmetry should be used, which predicts no dominant contribution for transition dipole moments. Albeit the point group theory can give us qualitative results, more sophisticated analysis are needed to obtain quantitative results. The important transition component for DSSCs is along the direction of the charge transfer, i.e. z-axis in current coordinates. Therefore, we dissected z-component of the

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transition dipole moment between S0 to S2 for molecule 1 and 2, as well as that between S0 to S1 for the molecule 3, 4, and 5 in Table 1. According to the involved MOs shown in Figure 4, all considered excited states are charge transfer states. Surprisingly, the major contribution for the transition dipole moment is not related to the transition from D to A, D to M, or M to A. Thus, the metallic atom merely serves as a bridge for the transition process. The small contribution between D and A should be attributed to that there is no effective overlap of MOs between them because of their separation by the metal atom. The transition dipole moment is mainly “borrowing” from the local excitation of D to D and A to A as shown in Table 1. In the molecules 4 and 5, the local D to D transition is dominant in the “borrowing” contribution, which is in agreement with the theory of Mulliken and Murrell. 11,12,47 This result may be attributed to the delocalized big π system of A which possesses a low electron ionization energy at its anionic form and can diffuse its wavefunctions to D through the metallic bridge at the excited state. However, for the molecules 1, 2, and 3, the local excitations of D to D and A to A have the similar contribution. This result should be attributed to the high electron affinity of the neutral form of A, which can induct electronic wavefunction from D at S0 . Therefore, to create a large transition dipole moment for the whole system, the complex should has large transition dipole moment of the local A to A and D to D transitions. Especially in the presence of a delocalized π system in A, it could be an easily accessible way to enhance the local D to D transition. Another interesting aspect for transition dipole moment is the switching between bright (allowed transition) and dark (forbidden transition) for the S1 . According to the Kasha’s rule, 48 fluorescence spectroscopy requires a compound with bright S1 while phosphorescence spectroscopy requires bright T1 and is governed by the rate of intersystem crossing. Owing to the competition between intersystem crossing and florescence channels, dark S1 is usually more favorable for phosphorescence spectroscopy. Because the frontier MOs of our model molecules are localized in either D or A (see Figure 4), changing the dihedral angle between D and A is a good way to tune the transition dipole moment between S0 and excited states.

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Figure 5: Calculated oscillator strength of S0 to S1 (circle dot) and S0 to S3 (square dot) for the molecule 4 along the dihedral angle between donor and acceptor at IEFPCM-B3LYP/631G(d) level. The black and red lines represent the corresponding fitted function for oscillator strength of S0 to S1 and S0 to S3 , respectively. For instance, tuning the dihedral angle between D and A in the molecule 4 from 0 to 90◦ can turn the S1 from bright to dark. Meanwhile, the S3 can be converted from dark to bright. Moreover, taking the inspiration from the twisted intramolecular charge transfer, 49 we can fit the relationship between oscillator strength and the dihedral angel by

f = a cos2 (bθ + c)

(3)

The fitting parameters a, b, and c for S1 are 0.20 a.u., 1.01, and -2.32◦, while for S3 are 0.14 a.u., 1.01, and 87.81◦ , respectively. The perfect match between Eq. 3 and calculated transition dipole moment shown in Figure 5 indicates that, in the Condon approximation, we can precisely control the transition probability between S0 and S1 by tuning the dihedral angle between D and A. In experiments, using different size of alkene group to substitute

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Table 2: Mulliken decomposition of frontier MOs (HOMO-1, HOMO, and LUMO) into M (Metallic Pt), A (Acceptor), and D (Donor) except the HOMO-1 of molecule 5. The largest contributions are showing in the bold font. MO HOMO-1

Fragment 1 2 3 4 M 0.22 0.16 0.21 0.19 A 0.05 0.03 0.02 -0.01 D 0.73 0.81 0.77 0.82

5 — — —

HOMO

M A D

0.19 0.08 0.14 0.02 0.01 0.05 0.79 0.91 0.81

0.15 0.09 0.05 0.02 0.80 0.89

LUMO

M A D

0.04 0.04 0.03 0.92 0.95 0.95 0.04 0.00 0.02

0.05 0.06 0.92 0.93 0.03 0.01

the hydrogen atoms on the benzene ring in the molecule 4 can be a practical way to tune the dihedral angle between D and A. The amount of transferred charge is also an important parameter for charge transfer states. The Mulliken analysis in Table 2 also reveals that all HOMO-1’s and HOMOs are localized in D and LUMOs are localized in A. Hence, all considered excited states are charge transfer states as their major excitation shown in Figure 4. The electron density changes of fragments between S0 and excited states that are most relevant to the first absorption band were listed in Table 3. For all molecules, the major component of charge transfer occurs between D and A, in line with an earlier assumption that all of the considered states are ligand to ligand charge transfer states. 50 The charge transfer between M and A is minor, which indicates that the metal atom does not hold the electron that transits from D to A. Furthermore, we found that the percentage of metal to ligand charge transfer in the molecules 4 and 5 is smaller than that in the molecules 1, 2, and 3. This result can be attributed to the fact that the delocalized π system of A in the molecules 4 and 5 can diffuse more portion of its wavefunction to M at excited state, which feedbacks the transferred electron from M to A.

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Table 3: Differential Bader charge in M (Metallic Pt), A (Acceptor), and D (Donor) between S0 and excited states. Fragment M A D a between b between

1a 2a 3b 4b 5b -0.13 -0.07 -0.08 -0.04 -0.02 0.65 0.67 0.62 0.61 0.65 -0.52 -0.60 -0.54 -0.57 -0.63 S0 and S2 S0 and S1

Conclusions Using DFT/TDDFT, we have studied the optical transition properties of five representative model Pt complexes. We found that, in the D-M-A systems, the energies of all frontier MOs (occupied and unoccupied) can be affected by the interaction between acceptor and Pt, while donor can only affect the energies of occupied MOs. A newly developed transition dipole decomposition method enabled us to find that the dominant contribution to the charge transfer transition is the local excitation of D to D transition and A to A transition. Moreover, for the complexes with acceptors having delocalized π system, the local transition of D to D is more important for the whole transition. The calculated results also showed that tuning the dihedral angle between A and D could control the brightness of S1 . According to the charge transfer of fragment, we conclude that the Pt complex with delocalized π system of A can maintain the oxidation state of metal during the excitation. By tuning the dihedral angle between D and A, the transition energy and probability can be precisely controlled so as to broaden the spectrum region of photo-absorption. All these findings could be useful for the design of dye in DSSCs as well as in fluorescence and phosphorescence spectroscopies.

Acknowledgement This work was financially supported by the National Basic Research Program (973 Program, No. 2014CB848900), the National Natural Science Foundation of China (Nos. 21421063 and 21473166), the Strategic Priority Research Program of the Chinese Academy of Sciences 15



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(XDB01020200), and Hefei Science Center CAS (2015HSC-UP011).

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Graphical TOC Entry

A

D

Pt e

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