Enhanced Two‐Photon Absorption of Organic Chromophores: Theoretical and Experimental Assessments

Functional organic materials with enhanced two‐photon absorption lead to new technologies in the fields of chemistry, biology, and photonics. In this article we review experimental and theoretical methodologies allowing detailed investigation and analysis of two‐photon absorption properties of organic chromophores. This includes femtosecond two‐photon excited fluorescence experimental setups and quantum‐chemical methodologies based on time‐dependent density functional theory. We thoroughly analyze physical phenomena and trends leading to large two‐photon absorption responses of a few series of model chromophores focusing on the effects of symmetric and asymmetric donor/acceptor substitution and branching.     

Magneto‐Photoluminescence Based on Two‐Photon Excitation in Lanthanide‐Doped Up‐Conversion Crystal Particles

Experimental studies on magneto‐photoluminescence based on two‐photon excitation in up‐conversion Y₂O₂S:Er, Yb crystal particles are reported. It is found that the up‐conversion photoluminescence generated by two‐photon excitation exhibits magnetic field effects at room temperature, leading to a two‐photon excitation‐induced magneto‐photoluminescence, when the two‐photon excitation exceeds the critical intensity. By considering the spin selection rule in electronic transitions, it is proposed that spin‐antiparallel and spin‐parallel transition dipoles with spin mixing are accountable for the observed magneto‐photoluminescence. Specifically, the two‐photon excitation generates spin‐antiparallel electric dipoles between ⁴S_3/2–⁴I_15/2 in Er³⁺ ions. The antiparallel spins are conserved by exchange interaction within dipoles. When the photoexcitation exceeds the critical intensity, the Coulomb screening can decrease the exchange interaction. Consequently, the spin–orbital coupling can partially convert the antiparallel dipoles into parallel dipoles, generating a spin mixing. Eventually, the populations between antiparallel and parallel dipoles reach an equilibrium established by the competition between exchange interaction and spin–orbital coupling. Applying a magnetic field can break the equilibrium by disturbing spin mixing through introducing spin precessions, changing the spin populations on antiparallel and parallel dipoles and leading to the magneto‐photoluminescence. Therefore, spin‐dependent transition dipoles present a convenient mechanism to realize magneto‐photoluminescence in multiphoton up‐conversion crystal particles.     

Polymer Microstructures through Two‐Photon Crosslinking

Two‐photon crosslinking of polymers (2PC) is proposed as a novel method for the fabrication of freestanding microstructures via two‐photon lithography. During this process in the confocal volume, two‐photon absorption leads to (formal) C,H‐insertion reactions, and consequently to a strictly localized crosslinking of the polymer. To achieve this, the polymer is coated as a solvent‐free (glassy) film onto an appropriate substrate, and the desired microstructure is written by 2PC into this glass. In all regions outside of the focal volume where no two‐photon process occurs, the polymer remains uncrosslinked and can be washed away during a developing process. Using a self‐assembled monolayer containing the same photoreactive group allows covalent attachment of the forming freestanding structures to the substrate, and thus guarantees an improved stability of these structures against shear‐induced detachment. As the two photon process is carried out in the glassy state, in a simple way, multilayer structures can be used to write structures having a varying chemical composition perpendicular to the surface. As an example, the 2PC process is used to build a structure from both protein‐repellent and protein‐adsorbing polymers so that the resulting 3D structure exhibits spatially controlled protein adsorption.     

Two‐Photon Vertical‐Flow Lithography for Microtube Synthesis

Two‐photon vertical‐flow lithography is demonstrated for synthesis of complex‐shaped polymeric microtubes with a high aspect ratio (>100:1). This unique microfluidic approach provides rigorous control over the morphology and surface topology to generate thin‐walled (<1 µm) microtubes with a tunable diameter (1–400 µm) and pore size (1–20 µm). The interplay between fluid‐flow control and two‐photon lithography presents a generic high‐resolution method that will substantially contribute toward the future development of biocompatible scaffolds, stents, needles, nerve guides, membranes, and beyond.     

Stereotactic Photodynamic Therapy Using a Two‐Photon AIE Photosensitizer

Two‐photon photodynamic therapy (TP‐PDT) is emerging as a powerful strategy for stereotactic targeting of diseased areas, but ideal photosensitizers (PSs) are currently lacking. This work reports a smart PS with aggregation‐induced emission (AIE) feature, namely DPASP, for TP‐PDT with excellent performances. DPASP exhibits high affinity to mitochondria, superior photostability, large two‐photon absorption cross section as well as efficient reactive oxygen species generation, enabling it to achieve photosensitization both in vitro and in vivo under two‐photon excitation. Moreover, its capability of stereotactic ablation of targeted cells with high‐precision is also successfully demonstrated. All these merits make DPASP a promising TP‐PDT candidate for accurate ablation of abnormal tissues with minimal damages to surrounding areas in the treatment of various diseases.     

Two‐Photon Up‐Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi‐2D Perovskite Films

A new approach to generate a two‐photon up‐conversion photoluminescence (PL) by directly exciting the gap states with continuous‐wave (CW) infrared photoexcitation in solution‐processing quasi‐2D perovskite films [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two‐photon up‐conversion PL occurring in quasi‐2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two‐photon up‐conversion PL signal. This confirms that the gap states are indeed responsible for generating the two‐photon up‐conversion PL in quasi‐2D perovskites. Furthermore, mechanical scratching indicates that the different‐n‐value nanoplates are essentially uniformly formed in the quasi‐2D perovskite films toward generating multi‐photon up‐conversion light emission. More importantly, the two‐photon up‐conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi‐photon excitation. Polarization‐dependent up‐conversion PL studies reveal that the gap states experience the orbit–orbit interaction through Coulomb polarization to form spatially extended states toward developing multi‐photon up‐conversion light emission in quasi‐2D perovskites.     

Lipid‐PEG‐Folate Encapsulated Nanoparticles with Aggregation Induced Emission Characteristics: Cellular Uptake Mechanism and Two‐Photon Fluorescence Imaging

Folate functionalized nanoparticles (NPs) that contain fluorogens with aggregation‐induced emission (AIE) characteristics are fabricated to show bright far‐red/near‐infrared fluorescence, a large two‐photon absorption cross section and low cytotoxicity, which are internalized into MCF‐7 cancer cells mainly through caveolae‐mediated endocytosis. One‐photon excited in vivo fluorescence imaging illustrates that these AIE NPs can accumulate in a tumor and two‐photon excited ex vivo tumor tissue imaging reveals that they can be easily detected in the tumor mass at a depth of 400 μm. These studies indicate that AIE NPs are promising alternatives to conventional TPA probes for biological imaging.