Nonlinear Optical Properties of Coordination Complexes

Nonlinear optical (NLO) properties of coordination complexes arise from the interaction between intense electromagnetic fields and molecular electronic structures, particularly those incorporating transition-metal centres coupled to π-conjugated organic ligands. The presence of metal ions enhances charge transfer and facilitates tunable polarisation responses, enabling both second-order effects such as second harmonic generation and electro-optic modulation, and third-order phenomena including multiphoton absorption and optical Kerr effects. Structural parameters—metal identity, oxidation state, ligand donor–acceptor character and molecular symmetry—govern hyperpolarizability and dictate whether dipolar, quadrupolar or octupolar arrangements prevail. Recent design strategies exploit dendritic architectures, polyoxometalate acceptors and chiral Schiff bases to augment nonlinear response and improve processability. Advances in computational methods, notably density functional theory, guide the rational engineering of electronic transitions and provide insight into structure–property correlations. Coordination complexes thus offer a versatile platform for photonic devices, including tunable frequency converters, optical switches, direct-writing photolithography and bioimaging probes operating in telecommunication and biological optical windows. Their global significance extends to the development of compact all-optical circuits, data encryption systems and high-resolution multiphoton microscopy.

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In the realm of high-order nonlinear absorption, systematically engineered organometallic dendrimers have demonstrated exceptional instantaneous three- to six-photon absorption cross-sections over a broad near-infrared spectral range. Incremental dendrimer generations and peripheral functionalisation not only amplified absorption efficiencies but also enabled wavelength tuning through substituent variation. Time-dependent density functional theory has elucidated the pivotal role of the metal core in mediating key electronic transitions. Complementary studies on mononuclear copper(II) Schiff base complexes with chiral tridentate or tetradentate ligands have revealed robust solid-state second-order responses, as measured by the Kurtz–Perry powder technique. The incorporation of asymmetrically substituted salicylidene moieties engenders significant charge asymmetry, yielding non-zero quadratic susceptibilities approaching half the efficiency of standard urea. More recently, hybrid porphyrin materials bearing ferrocene donors and fullerene acceptors linked via ethynyl spacers have highlighted the interplay of dipolar orientational and purely electronic cubic contributions in electric field-induced second harmonic generation measurements. Computational insights confirm that cubic terms can dominate over orientational ones in systems with near-zero ground-state dipole moments, underscoring the need to consider third-order effects in EFISH analyses.

Nonlinear Optical Properties of Coordination Complexes publication trend

The graph below shows the total number of articles in nonlinear optical properties of coordination complexes across all publications each year (not limited to Nature Index journals).

Technical terms

Coordination complex: A compound in which a central metal ion is bonded to surrounding ligands through coordinate covalent bonds.

Multiphoton absorption: The simultaneous absorption of two or more photons by a molecule, leading to electronic excitation at lower individual photon energies.

Hyperpolarizability: A measure of a molecule’s nonlinear response to an applied electric field, determining its efficiency in frequency doubling and higher-order optical processes.

Second harmonic generation (SHG): A nonlinear optical process in which two photons of the same frequency combine to produce one photon with twice the frequency.

Electric field-induced second harmonic generation (EFISH): A technique to assess second-order nonlinear response by applying a static electric field during measurement of SHG in solution.

Dendrimer: A highly branched macromolecule with tree-like architecture used to organise multiple chromophores and amplify nonlinear optical effects.

References

1. Exceptional three- to six-photon absorption at organometallic dendrimers. Chemical Science (2024).
2. Donor–acceptor organo-imido polyoxometalates: high transparency, high activity redox-active NLO chromophores. Dalton Transactions (2016).
3. Solid-State Nonlinear Optical Properties of Mononuclear Copper(II) Complexes with Chiral Tridentate and Tetradentate Schiff Base Ligands. Materials (2019).
4. Nonlinear Optical Properties of Porphyrin, Fullerene and Ferrocene Hybrid Materials. Materials (2021).
5. Electric-Field-Induced Second Harmonic Generation Nonlinear Optic Response of A4 β‑Pyrrolic-Substituted ZnII Porphyrins: When Cubic Contributions Cannot Be Neglected. Inorganic Chemistry (2020).