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Nonlinear optical materials are one of the key research objects in the field of optics, which mainly research the nonlinear effects of the interaction between luminesce and matter. Compared with inorganic nonlinear optical materials, organic nonlinear materials have outstanding advantages: strong adaptability, high flexibility, low cost, easy modification and damage resistance. In this review, the electric field induced second harmonic generation (EFISH) experimental technology is used to measure and research the nonlinearity of iridium metal complexes. And because of its structural diversity, people can design molecules according to their needs to get the best nonlinear optical response. Organic molecules with large nonlinear coefficients should have the following characteristics: asymmetric charge distribution, the delocalized nature of π electrons, and easy polarization by external electric fields, and a large π conjugated system. In recent years, metal organic compounds have become a leader in the field of optics, mainly because of their very good nonlinear optical properties. In the future, people will do more investigation on the nonlinearity of metal organic complexes. Researchers have shown great interest in iridium metal organic complexes due in particular to their attractive stability and nonlinear activity. This review mainly studies the nonlinear principle, performance test and Measurement of nonlinearity of iridium metal complexes. The nonlinear properties of other metal-metal organic complexes will not be discussed.

The nonlinear optical effects of urea picric acid dinitroaniline and other organic compounds were discovered at the beginning of the study of nonlinear optical materials. Since organic molecules with large non-localized π-conjugated electron systems exhibit strong photoelectric coupling characteristics, they can obtain high response values and relatively large optical coefficients. After the 1980s, organic nonlinear optical materials developed rapidly. Compared with inorganic materials, organic materials have the advantages of high nonlinear optical coefficient, fast response, easy modification, damage resistance, easy processing, and strong molecular variability [

The electric polarization P ( r , t ) is a prerequisite for the nonlinear optical effect. Under the laser high-intensity photoelectric field E ( r , t ) , the medium will produce a nonlinear electric polarization intensity P N L ( r , t ) , which is a power relationship with the incident photoelectric field [

p = α E (1)

α is the linear polarizability of a molecule or atom.

Laser is a light source with high intensity, excellent monochromatism and coherence. The polarization intensity of the medium under such strong light is no longer a simple linear relation with the incident light intensity, but is related to the higher order term of the light field intensity.

At this point, the polarized pM of the molecule can be expressed as:

p M = α E + β E 2 + γ E 3 + ⋅ ⋅ ⋅ (2)

β is the first-order molecular hyperpolarizability (second-order effect), and γ is the second-order molecular hyperpolarizability (third-order effect). The stronger of incident light intensity will lead to a stronger nonlinear effect. Molecules have non-central symmetry, which is one of the indispensable conditions for optical materials to have nonlinearity, so β must not be zero.

If the incident field is an alternating electric field, then:

E = E 0 cos ( ω t ) (3)

Substituting Equation (3) into Equation (2), and taking into account the vector properties of electric field and polarization [

P M = ∑ j α i j E j + 1 2 ∑ j k β i j k E j E k + 1 6 ∑ j k l γ i j k l E j E k E l + ⋅ ⋅ ⋅ (4)

The optical susceptibility is only briefly introduced, and more details are in the literature [

Obtained from the classic two-level model theory: β = β_{1} + β_{CT}, where β_{1} reflects the interaction energy of each substituent with the π skeleton; And β_{CT} shows the contribution of charge transfer, that is, considering the interaction between the ground state and the excited state, its expression:

β C T = e 2 m ℏ 3 ω n g f n g Δ μ n g ( ω n g 2 − 4 ω 2 ) ( ω n g 2 − ω 2 ) (5)

where f n g is the oscillator strength of the transition between the ground state and the excited state, and Δ μ n g is the difference between the electric dipole moments of the ground state and the excited state. The comparison between experimental test results and calculations shows that the β C T term can explain the nonlinear response of disubstituted polyolefins and isomers of nitroaniline. From this equation, Equation (5) shows that in order to have a high β C T value, a compound must have charge-transfer transitions at low energy and huge Δ μ ng and f n g values. The oscillator strength of the transition should be increased as much as possible and the difference between the excited state and the ground state dipole moment should be increased. Therefore, the most effective method is to increase the electron activity of the donor and acceptor substituents.

By calculating the nonlinear optical susceptibility, the nonlinear optical properties of a series of organic molecules synthesized in the laboratory are theoretically calculated and predicted. The relationship between molecular structure and optical properties is discussed. A large number of studies have verified that the value β of the molecular system is extremely sensitive to the geometric structure characteristics of the molecule, so it is necessary to determine the geometric structure of the molecule in order to calculate the nonlinear response of the molecular system.

The results from

Optical susceptibility is one of the important indicators for studying the quality of nonlinear optical materials. The simplest and most direct method to measure the optical susceptibility of materials is the electric field-induced second harmonic generation (EFISH) experimental technique [

chromophore | β v e c | β v e c ， 2 | β v e c , 3 | β vec,2 / β v e c , 3 | M % | C C I |
---|---|---|---|---|---|---|

6.7 | 9.6 | −2.9 | −3.3 | 100 | 0.96 | |

−11.4 | −24 | 12.6 | −1.9 | 73 | 0.97 | |

−1.3 | −2.4 | 1.1 | −2.2 | 60 | 0.77 | |

11.8 | 31.6 | −19.8 | −1.6 | 86 | 0.90 | |

14.6 | 34.6 | −20.0 | −1.7 | 81 | 0.92 | |

8.8 | 19.4 | −10.6 | −1.8 | 84 | 0.98 | |

31.4 | 67.7 | −36.3 | −1.9 | 88 | 0.87 | |

40.3 | 69.0 | −28.7 | −2.4 | 93 | 0.95 |

30.4 | 77.9 | −47.5 | −1.6 | 80 | 0.80 | |
---|---|---|---|---|---|---|

30.8 | 71.8 | −41.0 | −1.8 | 76 | 0.77 | |

−12.4 | −24 | 11.6 | −2.1 | 65 | 0.85 | |

26.2 | 128 | −102 | −1.3 | 75 | 0.74 |

• the unit of β: 4.2 × 10^{−}^{40} m^{4}/V, M: the percentage of a major excitation contributed. C C I : the mixing coefficient of interaction.

Using heat-insulating lead glass and interference filter to filter out the fundamental frequency light passing through the sample cell, and the frequency-doubled signal is received by a low-noise, high-sensitivity photomultiplier tube. When measuring quartz samples, a central attenuator must be added to prevent the signal from being too strong to reduce the sensitivity of the photomultiplier tube. The signal is averaged by the sampling integrator and recorded by the computer. In order to ensure the reliability of the relative ratio of light intensity, the quartz sample is retested at fixed intervals (about half an hour). It summarizes the whole process of EFISH experimental data processing (

γ E F I S H = ( μ β λ / 5 κ T ) + γ ( − 2 ω ; ω , ω , 0 ) (6)

μ β λ / 5 κ T : dipolar orientational contribution γ ( − 2 ω ; ω , ω , 0 ) : the electronic contribution to EFISH which is negligible for dipolar molecules is the projection along the dipole moment axis of the vectorial component of the tensor of the quadratic hyperpolarizability, working with an incident wavelength. µ is the ground state dipole moment. Extrapolation to zero frequency ( ∨ Λ = 0.0 eV ; λ = ∞ ) obtains the determination of μ β 0 where β 0 is the static quadratic hyperpolarizability, it is a key point for comparing the second-order NLO properties of molecules. The μβ_{0} value can be obtained by the following equation:

μ β 0 = μ β λ [ 1 − ( 2 λ max / λ ) 2 ] [ 1 − ( λ max / λ ) 2 ] (7)

λ and λ_{max} is the absorption wavelength. A molecule having a μ β 0 higher than that of Disperse Red One (450 × 10^{−48} esu), which can prepare stable NLO-active hybrid polymeric films [

1) quadratic hyperpolarizability of donor-acceptor molecules

The main feature of push-pull molecules is that they are conjugated by the

principal end group, which makes the molecules have attractive optical nonlinearity. The strong absorption of these molecules in the ultraviolet region and the large ground state molecular dipole moment are mainly due to the influence of intramolecular charge transfer (ICT). The significant dipole moment variation is caused by the transition from the ground state to the excited state, and the static secondary polarizability is obtained between the ground state and the excited state. Secondary hyperpolarizability plays an important role in intramolecular charge transfer [

As shown in _{CT} (0) and β_{max} (0) values using data from both absorption and electro-optical absorption measurements (EOAM) via a two-state bimorphological model [

n | MIX | β_{μ} (0) | β_{CT} (0) | β_{CT} (0) |
---|---|---|---|---|

1 | −0.49 | 32 | 35 | 36 |

2 | −0.65 | 97 | 96 | 129 |

3 | −0.73 | 210 | 200 | 368 |

4 | −0.76 | 405 | 288 | 600 |

The accuracy of the experimental values β_{μ} (0) is about 10%. The MIX and β_{max} (0) computed values obtained with the two-state two-form model. β_{CT} (0) from ref. [

but remained essentially the same except for long chains of n = 4, indicating that the quadratic approximation was suitable for push-pull polyolefins (_{max} (0) increased significantly, resulting in a significant increase in the second-order nonlinearity. This indicates that the growth of chain has a direct effect on the secondary hyperpolarimetry of these compounds.

2) Cyclometalated Ir (III) Complexes with Curcuminoid Ligands

It is well known that for pyridine-based iridium metal organic complexes, the free 2-phenylpyridine NLO response is very weak. Therefore, the metal iridium and the cyclometalization reaction play a decisive role in improving the nonlinear activity of such metal complexes. After studying a family of iridium (III) acetylacetonate compounds with various cyclometalated 4-styryl-2-phenylpyridines substituents (NEt_{2}, OMe, NO_{2}), In order to undestand that whether appropriate functionalization of 2-phenylpyridine ligands is beneficial to the improvement of secondary NLO activity [_{1.907} value of the 4-styryl-2-phenylpyridine bearing the nitro group, is comparable to 2-phenylpyridine [^{−3} M) by The Electric Field Induced Second Harmonic generation (EFISH) method. For complexes Ir_{1} and Ir_{2}, we choose a 1.907 µm as incident wavelength achieved by Raman-shifting the 1.064 µm wavelength obtained from a Q-switched, mode-locked Nd: YAG laser.

Obviously, the values of these two iridium metal complexes are negative. The µβ_{1.907} value of complex Ir_{1} is −1050 × 10^{−48} esu, −930 × 10^{−48} esu for complex Ir_{2} respectively. Some studies show that the µβ_{1.907} value of other cyclometalated iridium (III) complexes with β-diketonate is also negative [

For complex Ir_{1}, the µβ_{1.907} experimental value (−1050 × 10^{−48} esu) is comparable to µβ_{0} calculated value (−747 ×10^{−48} esu) by Equation (7). The low energy charge transfer absorption band (470 nm [_{2} with tetrahydrocurcumin as an ancillary ligand has a similar second-order NLO response (µβ_{1.907} = −930 × 10^{−48} esu; µβ_{0} = −661 × 10^{−48} esu by Equation (7) and λ_{max} = 470 nm [_{1.907} EFISH is the result of positive and negative contributions to the quadratic hyperpolarizability [

3) Iridium complexes with anellated hemicyanine ligands

The second-order NLO activity of anellated hemicyanine ligands [N,N-dimetyl-4-(pyridin-4-yl)aniline, L_{1} and N-methyl-N-hexadecylaminostilbazole, L_{2},7-N,N-dibutylamino-2-azaphenanthrene, L_{3}, and 8-N,N-dibutylamino-2-azachrysene, L_{4}. systems containing an electron-withdrawing substituent (-CN), L_{5}], and iridium(III) compounds with anellated hemicyanine ligands {cis-[Ir(CO)_{2}ClL_{3}], IrL^{3}, and cis-[Ir(CO)_{2}ClL_{4}], IrL^{4}} have been investigated (_{1.907} (EFISH) of iridium(III) compounds with anellated hemicyanine ligands are reported in ^{1} in trichloromethane solution present a low absolute value of μβ_{1.907} (EFISH) (48 × 10^{−48} esu), which can be attributed to the absence of a complete π-conjugation between the two aromatic rings. However the absolute value of μβ_{1.907} (EFISH) (L^{3}: 430 × 10^{−48} esu) is 8.9 times as much as L^{1}, which is due to include the single bond into the polyaromatic scaffold of L^{3}. It is quite clear that a continuous enhancement of the π-conjugation for the free ligand gives rise to a huge positive effect on the second order NLO response. Hence, the absolute value of μβ_{1.907} (EFISH) of the free ligands: L^{3} and L^{4} is much lagerer (430 × 10^{−48} esu, 1800 × 10^{−48} esu) than that of L^{2} (223 × 10^{−48} esu). Strangely enough, there is different to what was observed in the case of stilbazolium salts [^{4} to the second order NLO is weak, although generating a bathochromic shift of the ILCT transition. That is usually originated from the difference in the direction of the dipole moment，which governs the overall EFISH response. More surprisingly, the β values of L^{3} and L^{4} ligands are large and positive, which become decrease and even negative. When coordinated (

μβ_{1.907}^{a,b} (×10^{−48} esu) | μ^{c} (μ_{theor}) [D] | β_{1.907} (×10^{−30} esu) | |
---|---|---|---|

L^{1} | 48 | 2.4 | 20^{d} |

L^{2} | 223^{e} | 3.7^{e} | 60^{e} |

L^{3} | 430 | 3.5(7.6) | 123^{d}(56)^{e} |

[IrL^{3}] | 620 | (16.2) | (38)^{e} |

L^{4} | 1800 | 4.2(8.0) | 429^{d}(224)^{e} |

[IrL^{4}] | −2310 | (16.9) | (−137)^{e} |

^{a}EFISH values determined in CHCl_{3} with an incident wavelength of 1.907 mm. ^{b}The error on EFISH measurements is ±10%. ^{c}Values measured in CHCl_{3} by the Guggenheim method. ^{d}By using experimental µ. ^{e}Data from ref. [

Ligands | μβ_{1.907} (×10^{−48}esu)^{a} | Complexes | μβ_{1.907} (×10^{−48} esu)^{a} |
---|---|---|---|

L^{3} | 587 | [IrL^{3}] | 819 |

L^{5} | −1030 | [IrL^{5}] | −2350 |

L^{6} | −1830 | [IrL^{6}] | −1990 |

^{a}In DMF at 10^{−4} M with an incident radiation of 1.907 µm. The error on EFISH measurements is ±10%.

With this fundamental concern, it is easy to find that the µ of the ground state is lower than that of the excited state. This can be explained by two factors: ILCT transition (β > 0) and MLCT transition (β < 0). As shown in ^{2} to the same Ir(I) moiety where µβ_{1.907} is enhanced but remains positive, being dominated by an ILCT Transition [^{5}, is coordinated to the iridium fragment would be in agreement with the importance of the MLCT transition (

4) Cationic cyclometallated iridium (III) complexes with substituted 1,10-phenanthrolines

A family of Ir(III) complexes of [Ir(ppy)2(5-R-1,10-phen)] [PF6] (ppy = cyclometallated 2-phenylpyridine), (1a-b), [Ir(ttpy)_{2}(5-R-1,10-phen)] [PF_{6}] (ttpy = cyclometallated 3'-(2-pyridil)-2,2':5',2''-terthiophene, phen = phenanthroline; 2a-b, R = Me, NO_{2}) and [Ir(pq)_{2}(5-R-1,10-phen)] [PF_{6}] (pq = cyclometallated 2-phenylquinoline, 3a-b) was investigated (_{2}Cl_{2}. All compounds show a negative μβ_{1.907} (EFISH) (^{−48} esu to −2230 × 10^{−48} esu. Complex [Ir(ppy)2(5-R-1,10-phen)] [PF6] (ppy= cyclometallated 2-phenylpyridine) shows a large absolute value of μβ_{1.907} (EFISH), which presents a strong second order NLO response [_{2}(5-R-1,10-phen)] [PF_{6}] (pq = cyclometallated 2-phenylquinoline). Substitution of ppy with the

Absorption max (nm) | Emission max (nm) | Quantum yield Φ (%) | EFISH μβ_{1907}^{a,b} (10^{−30} Dcm^{5}esu^{−1}) | |
---|---|---|---|---|

1a | 255(sh), 268, 333(sh), 377^{c,d} | 559^{c} | 38^{c} | −1565^{e} |

1b | 254(sh), 264, 378^{c,d} | - | <0.1^{c} | −2230^{e} |

2a | 273, 329, 347(sh), 431^{c,d} | 556^{c} | 34 | −2090 |

2b | 268, 324, 430^{c,d} | - | <0.1^{c} | −1720 |

3a | 260, 319(sh), 396^{d} | 563 | 1.3 | −1320 |

3b | 250(sh), 264, 360(sh), 377, 407(sh)^{d} | - | <0.1 | −1640 |

^{a}EFISH measurements are carried out in CH_{2}Cl_{2} at 10^{−3} M; ^{b}The error of EFISH measurements is ±10%; ^{c}Ref. [^{d}There is a band tail above 400 nm up to about 500 - 550 nm; ^{e}Ref. [

more π-delocalized pq does not affect significantly the luminescence and NLO properties. A slightly lower NLO response and a much poorer luminescence is observed for the related complexes with ttpy. It is worthwhile mentioning that these complexes have unique characteristics in good transparency towards the second harmonic emission renders appealing as building blocks for composite second order NLO materials [_{2}Cl_{2}. Comparison of the properties of 2a-b with that of 1a-b puts in evidence that substitution of ppy with the more π-delocalized pq does not affect significantly the luminescence and NLO properties(

In this review, the nonlinear optical properties of iridium complexes were investigated, the relationship between their NLO activity and molecular structure was elucidated, and the great potential of iridium complexes in NLO optical materials was demonstrated. Nonlinear polarimetry is one of the main indexes to measure the quality of nonlinear optical materials, and its size is directly related to the molecular structure. Therefore, studying the molecular nonlinear polarimetry at the molecular level is of great help to design new and efficient nonlinear optical devices. There are several methods to determine the second-order nonlinear polarizability of molecules. At present, the most accurate and effective method is EFISH experimental method, namely EFISH(electric-field-induced second harmonic wave) method, which is used to determine the nonlinear polarizability in the direction of the molecular dipole moment. Which provides a bridge between micromolecules and molecular engineering, and points the way for synthetic chemists and materials scientists to find new nonlinear optical materials. In this paper, the working principle, experimental steps and calculation results of EFISH method are introduced in detail, and the results are discussed preliminarily. In this paper, the working principle, experimental steps and calculation of EFISH method are introduced in detail, and the results are discussed preliminarily. For most polyolefins, there is little correlation between hyperpolarization and geometric parameters. Nonetheless, for push-pull polyolefin, Maker and coworkers have been proved that in the molecular structure the band length alternation (BLA) and π-electron bond order alternation make a hugely positive effect on the hyperpolarizability [

Financial support from the National Natural Science Foundation of China (Grants 50925207, 51172100, 51432006, and 51602130), the Ministry of Science and Technology of China for the International Science Linkages Program (Grants 2009DFA50620 and 2011DFG52970), the Ministry of Education of China for the Changjiang Innovation Research Team (Grant IRT13R24), the Ministry of Education and the State Administration of Foreign Experts Affairs for the 111 Project (Grant B13025), 100 Talents Program of CAS, Jiangsu Innovation Research Team and Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (Grant 16KJD430002) are gratefully acknowledged.

The authors declare no conflicts of interest regarding the publication of this paper.

Li, L., Duan, H.J. and Qian, J. (2021) Review on the study of Nonlinear Optics of Iridium Metal Organic Complex. Journal of Materials Science and Chemical Engineering, 9, 24-42. https://doi.org/10.4236/msce.2021.92003