Hydrophobic Chromophores in Aqueous Micellar Solution Showing Large Two-Photon Absorption Cross Sections

Over the past few years, intensive studies have been focused on developing two-photon absorbing (2PA) materials that can be excited in the near IR region (700–1000 nm) because of their potential applications in frequency upconversion lasing, high-density optical storage, two-photon fluorescence microscopy (TPM), and photodynamic therapy. Until now, most of the 2PA materials with large 2PA cross sections (d, expressed in GM = 1 × 10 cm s photon molecule) that have been developed so far are based on the molecular motifs of either linear or quasi-linear D-p-D, D-p-A-p-D, A-p-D-p-A conjugates (D: electron-donating group, A: electron-accepting group, and p: conjugation pathway). Upon excitation, a substantial symmetric intramolecular charge redistribution takes place in these molecules, resulting in high 2PA cross sections. Although many of these 2PA materials have d value greater than 2000 GM, their poor solubility in water prevents them from being used in biological applications. For biological imaging, TPM represents a noninvasive method that possesses the advantages of reduced photobleaching, minimal phototoxicity, and excitation confinement exclusively to the focal plane. In order to obtain a good signal-to-noise ratio, the 2PA materials used in TPM should have large twophoton action cross sections (gd; g is quantum yield) in polar aqueous media. However, most of the commonly used watersoluble 2PA materials only exhibit d values that are less than 100 GM, which gives a gd value in the range of 1–50 GM. Recently, Bazan et al. have successfully developed a series of water-soluble 2PA materials based on 3D [2.2]paracyclophane as the core. These materials exhibit quite large d values, up to 700 GM (gd is about 300 GM) in water. They have also demonstrated that hydrophilic 2PA materials can be encapsulated into micelles of sodium dodecyl sulfate in water. Because most 2PA chromophores are hydrophobic, it would be ideal if a simple method could be developed to ensure 2PA materials are biologically compatible while maintaining high gd values in polar aqueous media. In the literature, there are several reports concerning the use of amphiphilic block copolymers to form micelles in water to incorporate hydrophobic drugs. However, little attention has been paid to incorporating 2PA materials into micelles. Only one earlier report from Prasad et al. has demonstrated the feasibility of encapsulating hydrophobic 2PA chromophores into poly(D,L-lactide-co-glycolide) to get aqueous dispersion of 2PA nanoparticles. Here, we report a simple method to bring hydrophobic 2PA chromo-