Lighting the Way to See Inside Two-Photon Absorption Materials: Structure–Property Relationship and Biological Imaging

Abstract

The application of two-photon absorption (2PA) materials is a classical research field and has recently attracted increasing interest. It has generated a demand for new dyes with high 2PA cross-sections. In this short review, we briefly cover the structure-2PA property relationships of organic fluorophores, organic-inorganic nanohybrids and metal complexes explored by our group. (1) The two-photon absorption cross-section (δ) of organic fluorophores increases with the extent of charge transfer, which is important to optimize the core, donor-acceptor pair, and conjugation-bridge to obtain a large δ value. Among the various cores, triphenylamine appears to be an efficient core. Lengthening of the conjugation with styryl groups in the D-π-D quadrupoles and D-π-A dipoles increased δ over a long wavelength range than when vinylene groups were used. Large values of δ were observed for extended conjugation length and moderate donor-acceptors in the near-IR wavelengths. The δ value of the three-arm octupole is larger than that of the individual arm, if the core has electron accepting groups that allow significant electronic coupling between the arms; (2) Optical functional organic/inorganic hybrid materials usually show high thermal stability and excellent optical activity; therefore the design of functional organic molecules to build functional organic-inorganic hybrids and optimize the 2PA properties are significant. Advances have been made in the design of organic-inorganic nanohybrid materials of different sizes and shapes for 2PA property, which provide useful examples to illustrate the new features of the 2PA response in comparison to the more thoroughly investigated donor-acceptor based organic compounds and inorganic components; (3) Metal complexes are of particular interest for the design of new materials with large 2PA ability. They offer a wide range of metals with different ligands, which can give rise to tunable electronic and 2PA properties. The metal ions, including transition metals and lanthanides, can serve as an important part of the structure to control the intramolecular charge-transfer process that drives the 2PA process. As templates, transition metal ions can assemble simple to more sophisticated ligands in a variety of multipolar arrangements resulting in interesting and tailorable electronic and optical properties, depending on the nature of the metal center and the energetics of the metal-ligand interactions, such as intraligand charge-transfer (ILCT) and metal-ligand charge-transfer (MLCT) processes. Lanthanide complexes are attractive for a number of reasons: (i) their visible emissions are quite long-lived; (ii) their absorption and emission can be tuned with the aid of appropriate photoactive ligands; (iii) the accessible energy-transfer path between the photo-active ligands and the lanthanide ion can facilitate efficient lanthanide-based 2PA properties. Thus, the above materials with excellent 2PA properties should be applied in two-photon applications, especially two-photon fluorescence m...

Near infrared two-photon-excited and -emissive dyes based on a strapped excited-state intramolecular proton-transfer (ESIPT) scaffold

“…It has been well documented that A–π–D–π–A-type fluorophores exhibit a large two-photon absorption cross section (TPACS). 31 , 35 To assess this feature, we conducted TPA measurements on 5 together with 1 and 7 as reference compounds ( Fig. 7 ).…”

“…Notably, a symmetric acceptor–π–donor–π–acceptor structure usually benefits efficient two-photon-absorption (TPA) properties. 31 This fact prompted us to aim at a more red-shifted NIR emission upon excitation with NIR light through TPA. To optimize the structure to this end, the key features should be ( 1 ) the electron-accepting ability of the terminal groups and ( 2 ) the hydrogen-bond-forming ability of the strap moiety.…”

Enhanced two-photon absorption of ligated silver and gold nanoclusters: theoretical and experimental assessments

“…43,44 Over the last decade, considerable efforts have been made to develop organic dye molecules, ranging from well-known push-pull dipolar molecular structures, to quadrupoles and complex molecular architectures with large two-photon absorption (TPA) for fluorescence sensing and biological imaging applications. 45,46 A significant effort has also been made in the past decade for developing organic molecules and semiconductor quantum dots with large two-photon absorption (TPA) 47 for applications in fluorescence sensing and biological imaging. 48 Although nonlinear optics has a long history in organic chemistry, the field of NLO in ligated noble metal nanoclusters is relatively new 49,50 , but fast-growing.…”

Red AIE‐Active Fluorescent Probes with Tunable Organelle‐Specific Targeting

“…The excitation source for two‐photon excitation was a femtosecond optical parametric amplifier (Coherent OPerA Solo) pumped by an amplified Ti:sapphire system (Coherent Legend Elite system) and then detected with a spectrometer (Acton SpectraPro‐500i) coupled to a CCD. The TPA cross‐section (δ) of the sample was calculated at each wavelength according to the following formula where δ is the cross‐sectional value, c is the concentration of the solution, n is the refractive index of the solution, F is the two‐photon excited fluorescence integral intensity of the light emitted at the excitation wavelength, and ϕ is the fluorescence quantum yield.…”