Significant Enhancement of Two-Photon Excited Fluorescence in Water-Soluble Triphenylamine-Based All-Organic Compounds.

Understanding water-soluble and environmentally friendly two-photon absorption (TPA) molecules benefits the design of superior organic complexes for biomedical, illumination, and display applications. In this work, we designed two triphenylamine-based all-organic compounds and explored the mechanism of enhanced TP fluorescence in water solutions for potential applications. Experimentally, we showed that adding protein into our TPA molecule solution can drastically boost the TP fluorescence. Numerical simulations reveal that the TPA molecules prefer to dock inside the protein complex. We hypothesize that the interaction between our triphenylamine-based all-organic compounds and water molecules lead to non-radiative decay processes, which prevent strong TP fluorescence in the water solution. Therefore, the protection by, for example, protein molecules from such interactions can be a universal strategy for superior functioning of organic TPA molecules. Further experiments and numerical simulations support our hypothesis. The present study may facilitate the design of superior water-soluble and environmentally friendly superior organic complexes.

Enhanced Two-Photon Absorption in Two Triphenylamine-Based All-Organic Compounds.

Two-photon absorption (TPA) enables the excitation of molecules by comparatively lower energy photons with longer penetration depth and higher spatial precision control, which advances the uses of organic molecules in various applications. In this work, we report two simple all-organic molecules C42H33N (compound 3) and C138H168N4 (compound 14) with strong TPA and fluorescent emission activity. Density functional theory calculations show that the enhanced oscillator strengths could be responsible for improved TPA and emission activity for compound 14 compared to those for 3. The degradation of C138H168N4 under focused laser illumination without circulation is almost negligible within 5 h, making it a candidate for laser dyes. Solid-state measurements confirm the presence of a direct band gap for electron transition that determines the channel to release the absorbed energy and functionality of the studied molecules. This work emphasizes that a high TPA cross-section and selectable energy relaxation (fluorescent emission or heat dissipation) are equally important to the design of advanced functional TPA molecules.

Acousto-optic Bragg imaging of biological tissue.

Acousto-optic Bragg imaging is a technique that uses the interaction of light with ultrasound to optically image obstructions in acoustical fields. Existing reports of acousto-optic Bragg imaging based on transmission of acoustic fields through obstructions exhibit strong acoustic impedance mismatches manifested by poor image quality and missing details of physical structures of obstructions. In this work, the image quality was improved to exhibit detailed physical structures of an object by using an improved Bragg imaging system described in Sec. III below. This paper investigates the possibility of extending an acoustic Bragg imaging technique in transmission mode to image animal or plant tissues; a small azalea leaf is used as an illustration in this case. The Bragg image produced clearly shows the veins of the vascular azalea leaf serving as a proof of concept for cost-effective potential application of acoustic Bragg imaging of biological objects in the medical field. Moreover, acousto-optic Bragg imaging is potentially harmless to biological cells and is sensitive to density and elastic variations in the tissue.

Optical Bragg imaging of acoustic fields after reflection.

Bragg diffraction of x-rays occurs when the rays interact with a crystalline lattice at the appropriate angle. Bragg diffraction of visible light occurs when the light interacts at the Bragg angle with an ultrasonic field of the appropriate frequency. (The spacing between acoustic condensations and rarefactions acts like the planes in an atomic lattice.) If a beam of light is Bragg diffracted by an ultrasonic beam that previously has passed through an object, an image of the structure of the object is made visible in the diffraction field of the optical beam since there is a one-to-one mapping of the ultrasonic field onto the diffraction order. In many acoustic Bragg imaging applications, the sound field must pass through the object which is to be imaged. Ultrasonic attenuation at the very high acoustic frequencies needed for Bragg imaging (typically approximately 25-30 MHz) severely limits the nondestructive testing (NDT) applications of traditional acoustic Bragg imaging. In this paper, a reflection-based application of acoustic Bragg imaging is discussed which may have useful industrial and biomedical NDT applications.

Experimental study of nonequilibrium fluctuations during free diffusion in nanocolloids using microscopic techniques.

We report quantitative experimental results regarding concentration fluctuations based on a small-angle light-scattering setup. A shadowgraph technique was used to record concentration fluctuations in a free-diffusion cell filled with colloids. Our experimental setup includes an objective attached to the CCD camera to increase the field of view. We performed two separate experiments, one with 20 nm gold and the other with 200 nm silica colloids, and extracted both the structure factors and the correlation time during the early stages of concentration fluctuations. The temporal evolution of fluctuations was also qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. We found that the correlation time versus wavenumber for gold nanocolloids is concave shaped, whereas, for silica colloids, it is convex shaped. The difference in correlation time behavior is not only due to the size of the particle, but also to possible plasmonic interactions in gold colloids.