Theoretical study of two-photon absorption properties of a series of platinum (II) acetylide complexes

Dyes and Pigments 105 (2014) 75e88

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Theoretical study of two-photon absorption properties of a series of platinum (II) acetylide complexes Dan Wang, Lu-yi Zou, Shuang Huang, Ji-kang Feng, Ai-min Ren* State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Liutiao Road 2#, Changchun 130023, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 December 2013 Received in revised form 24 January 2014 Accepted 3 February 2014 Available online 12 February 2014

Transition metal acetylide compounds have attracted enormous attention because of the extraordinary photophysical properties arising from the electronic interaction between the transition metal and the organic molecular fragment. In this study, the geometrical structure, electronic structure, one-photon absorption (OPA) and two-photon absorption (TPA) properties of a series of Dep0 eAepe[Pt]epeAep0 eD type and A0 ep0 eAepe[Pt]epeAep0 eA0 type platinum(II) acetylides were studied theoretically by using density functional theory (DFT) and Zerner’s intermediate neglect of differential overlap (ZINDO) methods for getting TPA materials possessing large TPA cross-section. Our analysis suggests that intramolecular charge transfer between center metal and p-conjugated organic fragment dominates in TPA transitions, in which metal center increases conjugation length in the direction of long ligand. This contribution provides detailed theoretical analysis of one- and two-photon absorption property of Platinum (II) acetylide compounds and an effective way for designing of platinum (II)/nickel (II) acetylide compounds possessing large TPA cross-section. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Platinum (II) acetylides Molecular structure One-photon absorption Two-photon absorption Two-photon absorption cross-section Charge transfer

1. Introduction Two-photon absorption (TPA) is one of the third-order nonlinear optical properties. In recent years, two-photon absorption materials have attracted increasing interest due to their potential applications in many different fields. Materials that exhibit large TPA cross section have been widely used in optical power limiting [1e3], threedimensional (3D) optical storage memory [4e6], photodynamic therapy [7,8], frequency upconverted lasing [9,10], two-photon laser scanning fluorescence imaging [11,12] and live cells and tissue imaging [13e16]. Two-photon absorption, simultaneous absorption of two lower-energy photons, results in initiation of the same photophysical processes as one high-energy photon absorbed. Taking advantage of two-photon absorption materials can avoid from photodegradation effects. Second, the quadratic dependence of TPA on intensity causes photochemistry reaction to take place in a small focal region, allowing for more control in microfabrication and imaging applications [17]. But practical two-photon absorption materials suitable for variant applications are very limited. Therefore, the improvement and the design of novel two-

* Corresponding author. Tel.: þ86 431 88499567; fax: þ86 431 88945942. E-mail address: [email protected] (A.-m. Ren). http://dx.doi.org/10.1016/j.dyepig.2014.02.003 0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

photon materials are very urgent. It has been found that materials possessing good two-photon properties have either an asymmetrical DepeA (donorepeconjugated group-acceptor) or a symmetrical DepeAepeD or AepeDepeA structural motif [18e23]. A generally accepted TPA material design strategy is to develop chromophores with large changes in polarization upon excitation [18]. A class of optical nonlinear compounds of transition metal acetylides, in which the rigidity and linear geometry of the alkynyl group has made it an attractive unit in the design of interesting complexes, has attracted much attention. Small-molecular transition metal acetylides and their dendritic structures have been shown to be good chromophores for optical power limiting (OPL). And these acetylides effectively compensate for the deficiencies of the deep colors, the poor solubility and the technical difficulties associated with fabricating OPL devices of phthalocyanines and porphyrins [24e28]. These organometallic complexes can show extraordinary photophysical properties due to the electronic interaction between transition metal and organic molecular framework [29,30]. Metals such as Re, Ru, Ir, Pt, Au, Ni, and Fe can exert a strong influence on the optical properties of a conjugated system via several mechanisms, including spin-orbit coupling induced singlettriplet mixing [31,32]. For colorless materials, Pt compounds can be of interest since they generally have less absorbance in the visible region [28,33]. Pt acetylides have high linear transmission in the

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D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

visible region and significant nonlinear absorption over a wide spectral region and so they are potentially suitable for OPL applications [34e36]. Cooper and Rogers reported TPA, excited-state absorption (ESA) and OPL of high-intensity light of some squareplanar platinum (II) acetylides [37e41]. Good optical power limiting performance can result from efficient resonant or nonresonant TPA and from ESA when the absorbance is greater than that of the ground state. ESA may involve absorption by a triplet state, which is reached by intersystem crossing (ISC) from an initially populated singlet state. The presence of a heavy metal atom (such as Pt) interacting with a conjugated carbon system in the electronically excited-state can favor ISC [42]. It is reported that platinum (II) diphosphine acetylides exhibit moderate two-photon absorption cross-section values and promising OPL properties [17,34,35,43e45]. Compared to those molecules, platinum (II) diacetylides stabilized with diimine ligands were expected to show different properties and more applicable fields. As reported in the literature for most of the platinum(II)

terpyridyl complexes, the low-lying absorption band in the UVevis absorption spectrum mainly arises from the metal-to-ligand charge transfer (MLCT) transition, possibly mixed with some LLCT character in case of the presence of an acetylide ancillary ligand. In these transitions, the lowest unoccupied molecular orbital (LUMO) has predominant contribution from the terpyridyl ligand, while the highest occupied molecular orbital (HOMO) is dominated by the platinum d-orbital in the case of MLCT transition and/or centered on the alkynyl ligand for the LLCT transition. Therefore, structural modification is pivotal for tuning the excited-state properties [46e 51]. However, to the best of our knowledge, there has been little attention to the TPA property of Platinum (II) diimine diacetylides. Li Qu et al. reported two Platinum (II) diimine diacetylides with broadband NLO absorption and relatively large TPA cross-section [52,53]. Based on this, in this work, we have designed a series of D-p0 -A-p-[M]-p-A-p0 -D type and A0 ep0 eAepe[M]epeAep0 eA0 type of Metal(II) diimine diacetylide molecules (M ¼ Pt, Pd, Ni), studied the influence of substituted groups on one-photon

Fig. 1. Structures and corresponding names of the studied molecules.

D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

absorption and TPA properties, analyzed the function of metal in process of one- and two-photon absorption of these molecules, and explored the structureeproperty relationship of this kind of molecules, and try to get the material molecules possessing good twophoton properties and with transparent character. 2. Theoretical methods The TPA process corresponds to simultaneous absorption of two photons. The TPA magnitude of an organic molecule, at optical frequency u/2p, can be characterized by the TPA cross section d(u). It can be directly related to the imaginary part of the second hyperpolarizability g(u; u, u, u) by Refs. [54,55]:

dðuÞ ¼

3Zu2 4 L Im½gðu; u; u; uÞ 2n2 c2 3 0

(1)

77

where g(u; u, u, u) is the third-order molecular polarizability, Zu is the energy of the incoming photons, c is the speed of light, and 3 0 the vacuum electric permittivity. n denotes the refractive index of the medium and L corresponds to the local-field factor. In the calculations presented here, the latter two quantities are set to 1 (isolated molecule in vacuum). The sum-over-states (SOS) expression to evaluate the components of the second hyperpolarizability gabgd (a, b, g and d refer to the molecular axes) is used in this study, the gabgd Cartesian components are given by Refs. [56,57]. The damping factor of excited state K (GK) in the SOS expression are set to 0.10 eV in the present work, this choice of damping factor is found to be reasonable on the basis of the comparison between the theoretically calculated and experimental TPA cross section [58]. And orientationally averaged (isotropic) value of g is evaluated, which is defined as:

Fig. 2. Optimized ground-state geometries of all molecules.

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D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

Table 1 NBO charge of molecules (unit: electron).

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 PE2

hgi ¼

D0 (A0 )

p0

A

p

D

p

A

p0

D0 (A0 )

0.00231 0.00332 0.00309 0.02253 0.00889 0.00734 0.00476 0.00289 0.00523 0.02238 0.06945 0.07492 0.00072 0.00581 0.01505 0.01773

0.04613 0.04656 0.04619 0.03988 0.01675 0.04712 0.04235 0.04319 0.04256 0.02102 0.06548 0.24801 0.04334 0.04290 0.03903 0.01005

0.15467 0.15238 0.16085 0.16377 0.08777 0.16091 0.15709 0.15162 0.15334 0.06560 0.11978 0.03470 0.15617 0.16720 0.15967 0.06653

0.36939 0.35611 0.35973 0.37275 0.40360 0.32418 0.36191 0.34952 0.35655 0.40044 0.35684 0.39756 0.32068 0.31835 0.32305 0.35192

0.95140 0.91734 0.94267 0.73558 0.93161 0.86143 0.96276 0.92176 0.94486 0.93475 0.96094 0.93755 0.86841 0.89643 0.91751 0.85232

0.36937 0.35613 0.36025 0.37274 0.40360 0.32428 0.36192 0.34954 0.35645 0.40044 0.35679 0.39757 0.32068 0.31814 0.32303 0.35192

0.15486 0.15253 0.16089 0.16394 0.08793 0.16096 0.15709 0.15162 0.15314 0.06559 0.11953 0.03471 0.15616 0.16693 0.15968 0.06653

0.04610 0.04655 0.04610 0.03987 0.01675 0.04713 0.04235 0.04319 0.04260 0.02102 0.06551 0.24801 0.04334 0.04294 0.03904 0.01005

0.00235 0.00333 0.00363 0.02248 0.00893 0.00732 0.00475 0.00291 0.00531 0.02237 0.06949 0.07493 0.00071 0.00586 0.01505 0.01773

 1 X giijj þ gijij þ gijji 15 i;j

i; j ¼ x; y; z

(2)

where after is taken into eqn. (1), and then the TPA cross section d is obtained. Generally, the position and relative strength of the two-photon resonance are to be predicted using the following simplified form of the SOS expression [eqn. (4)] [58]:

df

2 M2 M0k kn 2

ðE0k  E0n =2Þ G

þ

2 Dm2 M0n 0n

ðE0n =2Þ2 G

(3)

where Mij is the transition dipole moment from the state i to j; Eij is the corresponding excitation energy, the subscripts 0, k and n refer to the ground state S0, the intermediate state Sk, and the TPA final state Sn, respectively; Dm0n is the dipole moment difference between S0 and Sn. In this paper, the DFT/B3LYP/6-31G*/LanL2dz was primarily used to calculate molecular equilibrium geometry and NBO charge analysis by using the Gaussian 09 program [59]. Then, UVevisible (ground-state, one-photon absorption) spectrum was obtained by single and double electron excitation configuration interaction by using Zerner’s intermediate neglect of differential overlap (ZINDO) method and TDDFT/BHandHLYP/6-31g*/LanL2dz method. In TDDFT calculations the solvent effects were considered by the polarizable continuum model (PCM) in toluene. We found that both results have better agreements with experiment. And ZINDO method is not inferior to the TDDFT in the aspect of description of one-photon absorption. Thus, the calculated transition dipole moment and the corresponding transition energy can be used to predict the TPA properties using ZINDO method. Then, by using Eqns. (1)e(4), we calculated the second hyperpolarizability g and TPA cross section d. However, the resonance integrals for the central metal, bs, bp, and bd in transition metal complexes are important parameters that can be commonly adjusted [60e62]. bs and bp are set equal (“bsp”) and represent the amount of interaction between s and p orbitals of the metal and those of adjacent ligand atoms. The bd values represent interaction of the metal d orbitals with ligands. The more negative b, values represent a greater interaction between the corresponding metal orbital(s) and ligand orbitals. It is well-known that the ZINDO program does not include the default values of resonance integrals for metal Pt. It is reported that values of bsp for Ni have the range from 1 to 32 eV, and the values of bd for Ni ranges from 29 to 45 eV [60e62]. The values of bsp and bd parameters suitable for the single molecule could be determined by the closest agreement of the resulting calculated electronic absorption spectrum with experimental results. Therefore, we found

that the values of 1 and 28 eV respectively corresponding to bsp and bd for metal Pt make the calculated electronic absorption spectra of molecules PE2 and 10 by using ZINDO closest to the experimental UVevis spectra. At the same time, the timedependent density functional theory (TDDFT) method is also used to calculate the electronic absorption spectra; the results further confirmed the rationality of using the values of bsp and bd for metal Pt in ZINDO calculations. 3. Results and discussions 3.1. Molecular design and geometry optimization The molecules studied in this work and corresponding names and structures are shown in Fig. 1. Molecule 1 owns broadband NLO absorption and relatively large TPA cross-section [52]. Based on D0 e p0 eAepeDepeAep0 eD0 molecule 1, molecules 2, 3, 4, 5, 6, 14 have been designed by changing center Pt with Ni and Pd, end group D0 , bridge A, short ligand of center Pt and p-bridge, forming a series of D0 ep0 eAepeDepeAep0 eD0 molecules. DepeA type molecule also has good TPA property, so molecule 7 has been made by substituting end group triphenylamine D0 with end group 2,20 bithiophene A0 . Based on molecule 7, molecules 8, 9, 10, 11, 12, 13, 15 have been designed by changing center Pt with Ni and Pd, A0 end group, A bridge, short ligand of center Pt and p-bridge, forming a series of A0 ep0 eAepeDepeAep0 eA0 molecules. The molecule PE2 is usually considered to be a convenient benchmark because it

Fig. 3. Calculated frontier orbitals’ energy levels.

D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

79

Table 2 One-photon absorption properties calculated by ZINDO and TDDFT methods. EXPa

TDDFT

ZINDO

l/nm

lmax/nm

lmax/nm

f

Transition characteristics

500 [52]

484.58

499.4

2.0564

S0eS1

2

480.09

474.1

2.0350

S0eS1

3

485.88

503.3

1.9920

S0eS1

4

481.27

494.7

1.7243

S0eS1

5

364.08

352.9

4.8696

S0eS5

6

508.34

527.7

2.1816

S0eS1

7

522.61

554.7

2.6051

S0eS1

8

509.65

519.3

2.6083

S0eS1

9

598.67

536.4

2.5264

S0eS1

416.39

413.2

4.5486

S0eS1

11

468.33

488.6

2.1327

S0eS1

12

384.74

370.2

4.4215

S0eS3

13

532.36

564.6

2.6045

S0eS1

14

499.50

499.1

1.8917

S0eS1

15

520.12

530.1

2.4631

S0eS1

326.00

334.3

1.7952

S0eS4

Compound

1

10

PE2 a

414 [52]

355 [63]

HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO1 HOMO HOMO2 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO HOMO1 HOMO2 HOMO2

/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

LUMO LUMOþ1 LUMO LUMOþ1 LUMO LUMOþ1 LUMO LUMOþ1 LUMOþ2 LUMOþ3 LUMO LUMOþ1 LUMO LUMOþ1 LUMO LUMOþ1 LUMO LUMOþ1 LUMOþ2 LUMOþ1 LUMOþ1 LUMO LUMOþ1 LUMOþ1 LUMOþ2 LUMO LUMOþ1 LUMO LUMOþ1 LUMO LUMOþ1 LUMOþ3 LUMO

63.0% 25.5% 49.6% 35.5% 62.8% 26.9% 64.7% 30.1% 44.9% 31.5% 71.8% 17.4% 67.0% 23.3% 54.9% 34.5% 62.3% 28.0% 39.0% 31.7% 11.4% 65.9% 27.9% 37.5% 31.0% 70.0% 20.6% 63.6% 25.6% 59.7% 30.0% 73.3% 10.5%

lOmax of the one-photon absorption in toluene.

has been studied rather extensively; therefore, in this work, it was also studied for a comparative investigation. It is well-known that in order to obtain accurate one- and twophoton absorption spectra, optimized ground-state geometry is essential. In this work, the DFT/B3LYP/6-31G*/LanL2dz is primarily used to optimize molecular equilibrium geometry by using the Gaussian 09 program. As shown in Fig. 2, the optimized results show that PE2 is C2h symmetry, which agrees with reported by T.M. Cooper et al. [63]. Platinum is square-coordinated in all investigated platinum acetylides, the bond angle NePteN and CePteC are 180 . In concerned platinum acetylides in this study, all atoms in two long ligands are in a same plane (except methyls of 9,9-dimethyl9H-fluorene and end benzenes of triphenylamine). Two 4methylpyridines are in same plane, which is vertical to the long ligand plane. Because of similar electronic structure between Pd and Pt, the structures of molecule 2 and 8 with Pd center are same with other Pt acetylides; but the structures of Ni center molecule 3 and 9 are different with corresponding Pt acetylides.

in Fig. 1. As shown in Table 1, molecules 1, 2, 3, 4, 5 and 6 belong to D0 ep0 eAepeDepeAep0 eD0 type, and molecules 7, 8, 9, 10, 11, 12, 13, 14, 15 and PE2 are A0 ep0 eAepeDepeAep0 eA0 type of molecules. Comparing of D groups of molecules 1, 2, 3, 7, 8 and 9, the order of the electron-donating ability of center part is Pd < Ni < Pt. Comparing of D0 end groups of molecules 1 and 4, the electrondonating ability of triphenylamine as end group is smaller than that of phenylamine group. Comparing of A bridge, the electronaccepting ability of 2,1,3-benzothiadiazole as bridge part in molecules 4, 7, 11 is stronger than 9,9-dimethyl-9H-fluorene in

3.2. NBO charge analysis For a group in a molecule, that it is an electron donor or an electron acceptor not only depends on its own nature, but also is affected by other groups in the molecule. In order to clarify electron-donating or -accepting function of each fragment in platinum/palladium/nickel acetylide compounds in this study, NBO charge analysis has been performed on the basis of optimized geometries by Gaussian program. The results are listed in Table 1. For the sake of convenience in the following description, the studied molecules are divided into nine parts, just like molecule 1 showed

Fig. 4. The relationship between the imaginary part of the third-order optical susceptibility and the number of states for molecule 10.

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D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

molecules 5, 10, 12. As for end groups A0 of molecules 9, 10, 11 and 12, the electron-accepting ability of benzothiazole in 11, 12 is larger than of 2,20 -bithiophene in 9, 10. Observing D groups of molecules 1, 6, 7 and 13, donating-electron ability of Pt with 4-methylpyridine in 1, 7 is stronger than that of Pt with trimethylphosphine in 6, 13.

Comparing of molecules 1, 14, 7 and 15, ethylenyl p-bridged reduce the electron-donating ability of center and D0 end group, increase the electron-accepting ability of A0 end groups. The influence of different functional groups on optical properties of the studied molecules will be discussed in the next section.

Table 3 Two-photon absorption properties of studied molecules. Compound

lTmax/nm

dTmax/GM

Transition nature

1

676.4

5085.4

S0eS1eS9

626.8

8248.0

S0eS1eS14

2

621.2

6219.8

S0eS1eS13

3

678.3

4563.6

S0eS1eS9

4

650.5

6424.5

S0eS1eS12

5

673.8

144.2

S0eS5eS6

646.4

137.3

S0eS5eS7

720.8

7444.3

S0eS1eS6

636.5

13,680.1

S0eS1eS12

752.3

10,416.4

S0eS1eS8

698.9

6645.2

S0eS1eS12

8

706.1

9977.9

S0eS1eS8

9

720.8

9041.7

S0eS1eS11

10

708.5 (720) [52]

159.5 (394) [52]

S0eS3eS6

11

648.5

6529.4

S0eS1eS13

12

701.3

290.5

S0eS1eS6

13

772.0

9462.4

S0eS1eS7

701.3

9088.5

S0eS1eS10

682.7

4562.9

S0eS1eS5

628.7

7157.3

S0eS1eS10

15

725.9

7732.9

S0eS1eS6

PE2

624.9 (595) [36,64]

149.5 (235) [36,64]

S0eS5eS6

6

7

14

HOMO1 HOMO2 HOMO HOMO2 HOMO1 HOMO,HOMO HOMO2 HOMO HOMO1 HOMO3 HOMO2 HOMO3 HOMO1 HOMO4 HOMO HOMO1 HOMO1 HOMO4 HOMO HOMO1 HOMO2 HOMO HOMO HOMO2 HOMO1 HOMO,HOMO HOMO HOMO1 HOMO1 HOMO2 HOMO4 HOMO,HOMO HOMO,HOMO HOMO2 HOMO3 HOMO1 HOMO3 HOMO2 HOMO3 HOMO HOMO2 HOMO3 HOMO4 HOMO1 HOMO HOMO1 HOMO3 HOMO HOMO1 HOMO2 HOMO4 HOMO,HOMO HOMO,HOMO HOMO1 HOMO HOMO HOMO1 HOMO2 HOMO,HOMO HOMO2 HOMO1 HOMO HOMO HOMO1

/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

LUMO LUMOþ1 LUMOþ7 LUMOþ1 LUMOþ6 LUMOþ1,LUMOþ1 LUMOþ1 LUMOþ7 LUMOþ8 LUMO LUMOþ1 LUMOþ1 LUMO LUMO LUMOþ3 LUMOþ2 LUMO LUMO LUMOþ4 LUMO LUMOþ1 LUMOþ1 LUMOþ4 LUMOþ1 LUMOþ3 LUMOþ1,LUMOþ1 LUMOþ5 LUMO LUMOþ4 LUMOþ1 LUMO LUMOþ1,LUMOþ1 LUMO,LUMO LUMOþ1 LUMO LUMO LUMO LUMOþ1 LUMOþ3 LUMOþ3 LUMOþ3 LUMOþ1 LUMO LUMO LUMOþ3 LUMOþ1 LUMOþ2 LUMOþ3 LUMOþ2 LUMOþ1 LUMO LUMOþ1,LUMOþ1 LUMO,LUMO LUMO LUMOþ1 LUMOþ7 LUMOþ6 LUMOþ1 LUMOþ1,LUMOþ1 LUMOþ1 LUMO LUMOþ3 LUMOþ1 LUMO

26.0% 19.1% 17.4% 16.3% 11.8% 12.8% 15.9% 13.3% 12.5% 25.4% 18.6% 34.4% 18.9% 11.6% 37.9% 35.2% 46.5% 21.3% 20.2% 41.4% 11.0% 10.8% 21.6% 15.9% 10.9% 11.9% 18.4% 17.9% 17.8% 23.8% 13.5% 19.4% 13.9% 35.0% 20.4% 10.1% 34.5% 16.2% 41.0% 37.4% 11.3% 28.3% 11.8% 11.1% 58.4% 16.5% 10.7% 35.4% 32.2% 26.9% 12.8% 19.5% 14.9% 27.1% 18.7% 22.8% 17.8% 12.5% 10.2% 26.9% 18.5% 12.1% 39.9% 36.8%

ILCT MLCT, ILCT LL’CT, ML’CT MLCT, ILCT LL’CT MLCT, ILCT MLCT, ILCT LL’CT, ML’CT LL’CT ILCT, MLCT ILCT, MLCT ILCT, MLCT ILCT ILCT LL’CT, ML’CT LL’CT LL’CT LL’CT LL’CT, ML’CT ILCT MLCT ILCT, MLCT LL’CT MLCT ILCT ILCT, MLCT LL’CT, ML’CT ILCT, MLCT LL’CT ILCT, MLCT ILCT ILCT, MLCT ILCT, MLCT ILCT, MLCT ILCT ILCT ILCT, MLCT ILCT, MLCT LL’CT, ML’CT LL’CT, ML’CT LL’CT, ML’CT ILCT, MLCT ILCT ILCT LL’CT, ML’CT LL’CT LL’CT, ML’CT MLCT, ILCT ILCT MLCT, ILCT ILCT MLCT, ILCT MLCT, ILCT ILCT ILCT, MLCT LL’CT LL’CT MLCT MLCT MLCT, ILCT ILCT MLCT, ILCT MLCT, ILCT ILCT

D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

3.3. Electronic structure and one-photon absorption Electronic structures are fundamental for the interpretation and understanding of the absorption spectra. The HOMO-LUMO energy gap plays key role on tuning electronic spectra. The calculated frontier orbital energy levels (four occupied and four unoccupied orbitals and energy gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) are shown in Fig. 3. As shown in Fig. 3, the energy gaps between the HOMO and LUMO of molecules 1e15 are all smaller than that of PE2, and the energy gaps of A0 ep0 eAepeDepeAep0 eA0 type molecules 7, 8, 9, 12, 13 are all smaller than that of D0 ep0 eAepeDe peAep0 eD0 type molecules 1, 2, 3, 4, 5. In contrast to molecules 1 and 7, the HOMOeLUMO gaps of molecules 2, 3, 8 and 9 increase respectively, which center Pt is substituted with Ni and Pd. Comparing of molecules 1, 7, 14, 15, ethylenyl p-bridge decrease the energy gap in D0 ep0 eAepeDepeAep0 eD0 molecule and increase the energy gap in A0 ep0 eAepeDepeAep0 eA0 molecule. In both D0 ep0 eAepeDepeAep0 eD0 and A0 ep0 eAepeDepeAep0 eA0 molecules, the energy gaps of molecules with Ni and Pd center are higher than those with Pt center due to lowing the HOMO level of the Ni and Pd center; the energy gap will be increased by 9,9dimethyl-9H-fluorene owning weaker electron-accepting property and be decreased by trimethylphosphine ligand. In D0 ep0 eAe peDepeAep0 eD0 molecules, the phenylamine enlarges energy

81

gap; in A0 epeAepeDepeAepeA0 molecules, the benzothiazole also enlarges energy gap. OPA properties of all of the investigated molecules are calculated by using the ZINDO program on the basis of optimized geometric structures. At the same time, the TDDFT method was also adopted to calculate the OPA properties for the sake of comparison. The maximal OPA wavelengths (lmaxO), the corresponding oscillator strengths (f), and the main configurations and weights by the ZINDO and TDDFT are listed in Table 2, together with available experimental data. Comparing the results calculated by TDDFT and ZINDO, the wavelength differences between them are small. Both the maximal OPA wavelengths by the two methods are in good agreement with the experimental values. Therefore, a semiempirical ZINDO method is also appropriate for the studied molecules in this work, and the selection of resonance integral parameters is reasonable. A successful calculation for OPA properties by estimating and tuning the bsp and bd value in the ZINDO program is very helpful for calculating two-photon absorption. As shown in Table 2, OPA wavelengths of studied molecules are in range of 330 nm-570 nm, absorption spectra of molecules 5, 10, 12, PE2 are in ultraviolet band and other molecules’ absorption spectra are in visible light region. Most maximum OPA peaks are constructed by the first absorption transition dominated by the transitions from HOMO to LUMO and from HOMO1 to LUMOþ1. Comparing of 1, 2, 3, 7, 8 and 9, the OPA spectra of Ni and Pd center

Fig. 5. Two-photon absorption spectra for studied molecules (a), (b), (c).

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molecules show large blue shift relative to that of Pt center molecules, and blue shift degree of Pd center molecules are greater. For molecules 1, 6, 7 and 13, the electronic spectra of 6 and 13 with trimethylphosphine ligand show a bathochromic shift. For D0 ep0 e AepeDepeAep0 eD0 type of molecules 1, 4 and 5, OPA wavelength maximum of 4 shows hypsochromic shift relative to 1, and 5 also shows hypsochromic shift relative to 4, that is to say, phenylamine and 9,9-dimethyl-9H-fluorene lead OPA wavelengths hypsochromic shift. Comparing of 1 and 14, p-bridge ethylene doesn’t change the OPA positions. For A0 ep0 eAepeDepeAep0 eA0 type of molecules 7, 10, 11 and 12, the OPA spectra of molecules 10 and 12 having weak electron-accepting bridge show large blue shift relative to 7 and 11, in contrast to 7 and 11, the OPA spectra of molecules 11 and 12 possessing strong electron-accepting end group also show a blue shift. For 7 and 15, ethylene p-bridge leads OPA spectra hypsochromic shift. In a word, Ni and Pd center molecules and the molecules with 9,9-dimethyl-9H-fluorene bridge owing weak electron-accepting ability show blue-shifted OPA spectra in this study, trimethylphosphine ligand leads red-shifted spectra. Both strong electron-donating end group phenylamine in D0 ep0 eAepe DepeAep0 eD0 molecules and weak electron-accepting bridge benzothiazole, ethylene p-bridge in A0 ep0 eAepeDepeAep0 eA0 molecules make OPA wavelength hypsochromic shift, they will be a way to design of colorless materials. 3.4. Two-photon absorption property 3.4.1. Determination of the number of excited states Based on the SOS formula, Eq. (2) and three-state approximation (3), we calculated the third-order optical susceptibility and the TPA cross section dmax by using the ZINDO-SOS method. In the calculation process, 300 states were chosen. In order to ascertain whether 300 states is enough for the convergence of third-order optical susceptibility, we investigated the relationship between the imaginary part of the third-order optical susceptibility (Img) and the number of exited states for all the molecules studied. Here, we take the larger molecule 10 for example, when the number of excited states accumulates to 250 states, Img value of 10 has already converged as illustrated in Fig. 4. All concerned molecules in this work were studied in the same way, and the results showed that all the Img were converged before 300 states.

3.4.2. The relationship between ground state charge distribution and TPA cross-section The maximum TPA wavelengths (lTmax), the maximum TPA cross-section (dTmax), and corresponding transition nature were collected and listed in Table 3. In addition, two-photon absorption spectra in the incident wavelength range of 600e1000 nm were shown in Fig. 5. As shown in Table 3, the calculated lTmax are in good agreement with the experimental results; while, the calculated dTmax are smaller than experimental observation values on the whole. However, for molecule 1, the lTmax is 800 nm, the dTmax value is 675GM in the result of experiment; Our calculation results show that the lTmax are 676 nm and 627 nm, respectively, and the corresponding dTmax value are 5085GM and 8248GM. The disagreement of two-photon absorption cross-section values between the calculation and experimental possibly be caused by the difference of measured wavelength range. The underestimation of dTmax in the experiment can be mostly attributed to the fact that the dTmax value measured at 800 nm is off-resonant maxima, and therefore it is difficult to compare them directly with the calculated values. Our calculation results will reflect the resonance two-photon absorption in the wavelength range of 600e1000 nm for the studied compounds in this study. As shown in Table 3 and Fig. 5, molecule 1, 5, 6, 7, 13 and 14 have two TPA peaks in range of 610e760 nm, other molecules only have one TPA peak. Comparing with A0 ep0 e AepeDepeAep0 eA0 type molecules, TPA spectra of D0 ep0 eAepe DepeAep0 eD0 type molecules show hypsochromic shift, and decreased dTmax value. For the molecules of 1, 2, 3, 7, 8 and 9, the order of lTmax is: lTmax of Pd center molecules < lTmax of Ni center molecules
Table 4 Net charge change DQ of every fragment of studied molecules in TPA process. Compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 PE2

Final state

S9 S14 S13 S9 S12 S6 S7 S6 S12 S8 S12 S8 S11 S6 S13 S6 S7 S10 S5 S10 S6 S6

DQ D0 (A0 )

p0

A

p

D

p

A

p0

D0 (A0 )

0.135 0.124 0.089 0.108 0.111 0.009 0.006 0.145 0.097 0.073 0.203 0.141 0.144 0.002 0.107 0.011 0.017 0.005 0.133 0.081 0.117 0.052

0.003 0.014 0.007 0.006 0.051 0.029 0.010 0.025 0.006 0.025 0.032 0.001 0.006 0.002 0.026 0.023 0.022 0.001 0.002 0.007 0.017 0.024

0.225 0.222 0.112 0.162 0.278 0.014 0.243 0.298 0.207 0.172 0.333 0.238 0.232 0.105 0.225 0.122 0.063 0.039 0.231 0.171 0.157 0.038

0.078 0.078 0.056 0.052 0.099 0.049 0.190 0.111 0.086 0.080 0.090 0.086 0.077 0.275 0.090 0.161 0.049 0.344 0.335 0.077 0.105 0.055

0.027 0.013 0.053 0.021 0.045 0.085 0.900 0.039 0.039 0.014 0.018 0.015 0.034 0.767 0.006 0.496 0.019 0.755 0.026 0.013 0.163 0.022

0.080 0.078 0.054 0.060 0.100 0.049 0.190 0.110 0.088 0.080 0.090 0.085 0.079 0.275 0.092 0.161 0.049 0.344 0.333 0.077 0.105 0.055

0.225 0.221 0.103 0.198 0.299 0.014 0.244 0.300 0.211 0.172 0.333 0.233 0.238 0.105 0.236 0.122 0.063 0.039 0.224 0.170 0.157 0.038

0.003 0.014 0.007 0.007 0.054 0.029 0.010 0.025 0.005 0.025 0.032 0.001 0.007 0.002 0.027 0.023 0.022 0.001 0.002 0.007 0.017 0.024

0.136 0.123 0.084 0.131 0.117 0.009 0.006 0.144 0.096 0.073 0.203 0.144 0.148 0.002 0.112 0.012 0.017 0.005 0.131 0.080 0.116 0.052

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NBO analysis) is smaller than molecule 1 and 7. There seems to be a contradictory with above conclusion that the stronger ICT results in the larger dTmax value, which will be explained in 3.4.3 section. Comparing D0 ep0 eAepeDepeAep0 eD0 type molecules 1 and 4, TPA wavelengths of molecule 4 with strong electron-donating end groups show a bathochromic shift and decreased dTmax (relative to the maximum TPA peaks of 626.8 nm and cross-section of molecule 1). NBO analysis shows that, although molecule 4 owning strong electron-donating end groups, its electron-donating ability is weaken in contrast to 1, lowered ICT, thus a decreased dTmax. To compare molecule 4 TPA spectra of molecule 5 with weak electronaccepting 9,9-dimethyl-9H-fluorene bridge shows a red shift, and its dTmax value is decreased by an order of magnitude. The structures of two molecules have no obvious difference, NBO analysis shows that, electron-donating ability of center Pt of molecule 5 is stronger than that of molecule 4, but charge transfer between A bridge and end groups in molecule 5 is much smaller than molecule 4, charge transfer of molecule 5 in total is smaller than that of molecule 4, so dTmax is smaller than that of molecule 4. For molecule 1 and 14, the ethylene p-bridge weakens the electron-donating ability of center and end group, and decreases the dTmax of molecule 14. The

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calculated TPA properties for A0 ep0 eAepeDepeAep0 eA0 type molecule 7, 10, 11 and 12 showed that the order of dTmax values is: d10 < d12 < d11 < d7. 9,9-dimethyl-9H-fluorene fragment makes the dTmax of 10 and 12 decrease by an order of magnitude than those of 7 and 11. It is reasonable, because of that according to the results of NBO analysis above, the electron-donating ability of center Pt and electron-accepting ability of A bridge in molecules 10 and 12 are weaker, which lead to a weak ICT and decreased dTmax value. TPA spectra of molecule 11 and 12 with benzothiazole end group show a blue shift relative to molecules 7 and 10, dTmax of molecule 11 is an half of that of molecule 7 and dTmax of molecule 12 is two times of molecule 10. In summary, molecules 1, 3, 4, 11 have large OPA in 460e480 nm and large TPA in 618e676 nm, therefore they will be excellent optical power limiting material. Generally speaking, TPA spectra of D0 ep0 eAepeDepeAep0 eD0 type molecule show a blue shift and TPA across sections are slight smaller than A0 ep0 eAepeDepeAep0 eA0 type molecule. To compare with the molecules with Pt center, the molecules with Ni center show a fairly lTmax and slightly small TPA across sections. Compare of Pt center molecule with 4-methylpyridine, Pt center molecules with trimethylphosphine ligand have larger lTmax and

Fig. 6. Contour surfaces of the frontier orbitals relevant to TPA for studied molecules (a), (b), (c).

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dTmax. The dTmax will be decreased an order of magnitude by using 9,9-dimethyl-9H-fluorene as bridge moiety. The ethylene p-bridge also decreases its dTmax value. For D0 epeAepeDepeAepeD0 type molecules, inducing phenylamine decreases dTmax value. For A0 e p0 eAepeDepeAep0 eA0 type molecule, when the A bridge is 9,9dimethyl-9H-fluorene, end group benzothiazole increases dTmax; when the A bridge is 2,1,3-benzothiadiazole, benzothiazole end group decreases dTmax. 3.4.3. The relationship between the net charge change and dTmax Since the NLO response is correlated to ICT during the excitation, the net charge change (DQ ¼ Qn  Q0) corresponding to main TP transitions in TPA process were calculated and listed in Table 4. Qn denotes Mulliken charge distribution of molecule in TPA final state; Q0 denotes Mulliken charge of molecule in the ground state. It can be concluded from Table 4 that the more net charge change DQ leads to the larger dTmax value for a molecule. And DQ can show number and direction of charge transfer accurately, providing the accurate estimation for dTmax. It can explain the contradiction between weaker electron-donating ability and larger TPA cross section in molecules 6 and 13. As shown in Table 4, the DQ values of A

bridge in molecules 5, 10 and 12 are positive and those of other molecules are negative, the DQ values of center D in these three molecules are negative, suggesting that in the TPA process, charge transfer is from long ligand (including A bridge) to center D group in these three molecules, and other molecules are reverse. It can be seen from Table 4, TPA cross sections of these three molecules are much smaller than that of other molecules, which suggested that the charge transfer from long ligand to center D group make center D and long ligand A lose their intrinsic donating and accepting electron character, so is adverse to enhancing TPA cross section. 3.4.4. The relationship between transition character and dTmax In order to further understand how the molecular structures affect the frontier molecular orbitals and confirm the internal factors which can affect TPA properties, we drew the molecular orbitals related to main transitions in TPA process using the GAUSSIAN VIEW software [50]. As shown in Fig. 6, the electron clouds locate on the occupied orbitals orderly, they are well proportionally distributed on the center Pt and long ligands. Also, the electron clouds on all the unoccupied orbitals are located in A bridge of long ligands or short ligand. So the transition characters

Fig. 6. (continued).

D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

are primarily assigned as MLCT mixed ILCT or ML’CT mixed LL0 CT. (ILCT means charge transfer from bonding orbital to antibonding orbital of long ligands, LL0 CT means charge transfer from long ligands to short ligands, MLCT means charge transfer from center metal to long ligands, ML0 CT means charge transfer from center metal to short ligands.) For example molecule 1, the weaker TPA (at 674.6 nm) comes from the electronic excitation of S0/S9, which can be described as a mixture of configurations HOMO1 / LUMO, HOMO2 / LUMOþ1. The corresponding transition characters are primarily assigned as MLCT and ILCT. It should be worth noting that the TPA transition characters of molecules 5, 10 and 12 which owning smaller TPA cross section are assigned as LL0 CT and ML0 CT, other molecules’ main transition characters are ILCT, MLCT (some also including LL0 CT and ML0 CT, because the electron clouds on the higher unoccupied orbitals are located in short ligands). Compare of molecule 5, 10 and 12 and other molecules, weak electronaccepting ability lead to that the electron clouds on all the unoccupied orbitals are located in short ligand and LL’CT and ML0 CT occur instead of ILCT and MLCT. In the view of molecular structure, two long ligands are in a same plane, two short ligands are in another plane, which is vertical to the long ligand plane. So short ligands and long ligands are non-conjugated; LL0 CT or ML0 CT has much less contribution to the increase of TPA cross section, therefore, the TPA cross-sections of molecules 5, 10 and 12 are smaller.

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For other molecules, ILCT and MLCT are main character of TPA transition, charge transfer between center metal and p-conjugated organic fragment dominated in TPA transitions, in which center metal increases conjugation length in the direction of long ligand, results in large TPA cross section. In one word, the properties of ligands can change electron cloud distribution of the unoccupied orbitals, and transform the charge transfer direction, and affect the TPA cross-sections. 3.4.5. The transition dipole moment and dTmax Next, calculated transition dipole moments will provide the level of ICT on qualitative. In the SOS model, the transition dipole moment and transition energy are the important influencing factors on the TPA cross-section. The transition dipole moment and transition energy between the ground state and CT states are given in Fig. 7. According to equation (3), dTmax is in direct proportion to transition dipole moments M20k and M2kn and in inverse proportion to energy detuning term (E0k  E0n/2)2. In order to clearly present the relation between M0k, Mkn, E0k, E0n and dTmax, the relationship between dTmax and X (M20kM2kn/(E0k  E0n/2)2) in TPA process was depicted in Fig. 8. As shown in Fig. 8, the plots of dTmax versus X are averagely distributed along the fitted line, suggesting a rough linear relationship, and simultaneously proving that three states expression can forecast TPA cross section and M0k, Mkn, E0k, E0n are

Fig. 6. (continued).

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D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

important elements to dTmax. It can be found from Figs. 6 and 7, center Ni decreases M0k, Mkn and dTmax; trimethylphosphine ligand increases M0k, Mkn and dTmax. 9,9-dimethyl-9H-fluorene leads M0k and Mkn decrease by an order of magnitude and E0k increase, resulting in a smaller dTmax. 3.4.6. The oscillator strength (f) and dTmax The above results show that changing the center atom and its ligand, A bridge and end group can greatly change position of maximum TPA peak and the transition dipole moments, thus the dTmax values are also effectively changed. Meanwhile, there is some relation between the oscillator strength (f) and the transition moments. To further investigate the relation between them is necessary. Fig. 9 shows the dependence of dTmax on the product of the f0k and fkn (f0k, oscillator strength of the transition from the ground state to intermediate state; fkn, oscillator strength of the transition from the intermediate state to final state in the TPA process). It can be found that for D0 ep0 eAepeDepeAep0 eD0 type molecules, the product of the f0k and fkn versus the dTmax present a well linear dependence. For A0 ep0 eAepeDepeAep0 eA0 type molecules, the plots of dTmax versus the product of the f0k and fkn are averagely distributed along the fitted line. Thus, we can deduce that the dTmax and the product of f0k and fkn are proportional. This result also implies that f is another important impact factor to dTmax. This proportional relation is reasonable, because of that the oscillator strength (f) is dictated as the following equation (4).

fA/B ¼

  8mp2 DEA/B jXA/B j2 þ jYA/B j2 þ jZA/B j2 2 3h

(4)

Fig. 7. Scheme of the calculated transition dipole moments and energy levels in TPA process (a), (b).

Fig. 8. The relationship between dTmax and X of studied molecules.

In the above h is Planck’s constant, m is the mass of the electron,

DEA/B is the energy difference of two states, and XA/B, YA/B, ZA/B are the transition electric dipole moments from state A to state B. Based on the equation, oscillator strength f is in direct proportion to square of transition dipole moments. According to three states expression, direct proportion relationship between dTmax and the product of f0k and fkn is obtained when the energy detuning term (E0k  E0n/2)2 has no change or little change. As shown in Fig. 7, E0k and Ekn have a little change in D0 ep0 eAepeDepeAep0 eD0 type molecules, means the change of energy detuning term (E0k  E0n/ 2)2 is small, so dTmax depends on M20kM2kn. Therefore, our calculation results show that the plots of the product of the f0k and fkn versus

Fig. 9. The relationship between dTmax and f 0kf kn (a), (b).

D. Wang et al. / Dyes and Pigments 105 (2014) 75e88

the dTmax present a well linear relationship for D0 ep0 eAepeDepe Aep0 eD0 type molecules (Fig. 9a). For A0 ep0 eAepeDepeAep0 e A0 type molecules, energy detuning term (E0k  E0n/2)2 changes much with the change of E0k and Ekn, so dTmax is influenced by both M20kM2kn and (E0k  E0n/2)2. The relationship between the product of the f0k and fkn and the dTmax isn’t completely in a fitted line for A0 e p0 eAepeDepeAep0 eA0 type molecules. 4. Conclusions The geometrical structure, electronic structure, one- and twophoton absorption spectra, TP transitions and TPA cross-sections of a series of Dep0 eAepe[Pt]epeAep0 eD type and A0 ep0 eAe pe[Pt]epeAep0 eA0 type platinum(II) acetylides have been explored in detail by using DFT, ZINDO programs and SOS equation. Calculated results show that Pt center is a well electron donor, these molecules have OPA peaks in range of 330e570 nm and have obvious TPA peaks in range of 610e760 nm. The maxima dTmax of D0 ep0 eAepeDepeAep0 eD0 type molecules are smaller than A0 e p0 eAepeDepeAep0 eA0 type molecules. Through the analysis of factors influenced TPA cross-section, it is concluded that: (1) dTmax values are decreased in the order of Pt, Pd and Ni compounds, while the metal palladium has the outstanding contributions on one- and two-photon absorption spectrum blue shift. (2) Charge transfer between center metal and p-conjugated organic fragment dominates in TPA transitions, in which center metal increases conjugation length in the direction of long ligand, results in large TPA cross section. (3) As a ligand, trimethylphosphine can cause a red shift of the maximum one-photon absorption wavelength (lOmax) and increased TPA cross-section (dTmax) on keeping the maximum TPA wavelength (lTmax) unchanged. (4) 9,9-dimethyl-9H-fluorene as A bridge make the OPA spectra hypochromatic shift and dTmax decrease sharply, because M0k and Mkn decrease by an order of magnitude and E0k rises rapidly. So weak electron-accepting group 9,9-dimethyl-9H-fluorene is not a good A bridge for obtainment TPA materials, but 2,1,3-benzothiadiazole is in this system. (5) Comparing of triphenylamine, the stronger electron-donating group phenylamine leads red-shifted TPA spectra and small dTmax in D0 ep0 eAepeDepeAep0 eD0 type molecules. Therefore, for D0 e p0 eAepeDepeAep0 eD0 type molecules, using triphenylamine end group, 2,1,3-benzothiadiazole bridge and trimethylphosphine ligand can get a large dTmax, for example molecule 6. (6) For A0 ep0 e AepeDepeAep0 eA0 type molecules, 2,20 -bithiophene make TPA spectra red shift and dTmax increase of molecules with 2,1,3benzothiadiazole bridge. So 2,20 -bithiophene is considered a good end group. For A0 ep0 eAepeDepeAep0 eA0 type molecules, using 2,20 -bithiophene end group and 2,1,3-benzothiadiazole bridge can receive a larger TPA cross-section, 4-methylpyridine and trimethylphosphine are good ligands, such as molecules 7 and 13. This paper is hoped to provide an effective way to design platinum (II) acetylides compounds possessing large TPA cross-section. Acknowledgments This work is supported by the Natural Science Foundation of China (No. 21173099 and 20973078), the Major State Basis Research Development Program (2013CB 834801), special funding to basic scientific research projects for Central Colleges. References [1] Calvete M, Yang GY, Hanack M. Porphyrins and phthalocyanines as materials for optical limiting. Synth Met 2004;141:231e43. [2] Spangler CW. Recent development in the design of organic materials for optical power limiting. J Mater Chem 1999;9:2013e20.

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