The interlayer space of clays is an interesting microenvironment to control the properties of included organic materials. In this study, chromic hybrid films consisting of synthetic saponite (SSA) and fluorescent organic molecules with planar p-conjugated systems were fabricated. The hybrid films exhibited reversible fluorescence color switching induced by swelling and drying, which caused the gallery height of SSA to change. Ultraviolet-visible absorption, steady-state and time-resolved fluorescence measurements of the hybrid films strongly suggested that the organic molecules formed an excimer in the swollen interlayer space of SSA, while they were a monomer in the dried interlayer space. The different emission wavelengths of the excimer and monomer resulted in the fluorescence color switching of the hybrid films. The reversibility of the color switching was maintained for at least 50 cycles with no change of fluorescence maxima.These chromic hybrid films represent a novel approach to obtain color switching.

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We synthesized two-photon absorption probes based on naphthalene and anthracene: 1,1¤-dimethyl-4,4¤-(2,6-naphthylenedi-2,1-ethenediyl)dipyridinium diiodide (NP) and 1,1¤-dimethyl-4,4¤-(2,6-anthrylenedi-2,1-ethenediyl)dipyridinium diiodide (AC). These two probes were successfully accumulated in mitochondria with different fluorescence colors: yellow (NP) and red (AC). Two-photon absorption cross sections (· (2) ) were 712 and 685 GM (1 GM = 10 ¹50 cm 4 s photon ¹1 molecule ¹1), respectively. These values are nine times larger than the · (2) of the commercially available mitochondria-selective probe with efficient two-photon absorption property (rhodamine 123).Two-photon fluorescence microscopy (TPFM) is known as an advanced technology for three-dimensional (3D) imaging of tissues and organs.1 This technique enables 3D imaging of blood flow, 2 neural network, 3 neutrophil movement, 4 local oxygen concentration in the bone, 5 and so forth. In a typical TPFM, the excitation wavelength is ca. 800 nm and the observed fluorescence is between 400 to 550 nm. Transparency for biological tissues in the wavelength region from 400 to 550 nm is insufficient. Therefore, imaging at a point deeper than 1 mm cannot be achieved using the conventional TPFM probe. In contrast, a wavelength region from 600 to 1300 nm is known as the "tissue optical window".6 Biological materials usually have low absorption in this window, so that light penetrates deeper inside the tissue. By employing the probe that can be both excited and emitted in the tissue optical window, deeper observation of tissue and organs by TPFM can be achieved.We have developed an efficient two-photon fluorescence probe: 1,1¤-dimethyl-4,4¤-(9,9¤-diethyl-2,7-fluorenediyl-2,1-ethenediyl)dipyridinium diperchlorate (FLW, Figure 1a). This probe comprises a fluorene core and pyridinium groups. 7 The cationic pyridinium moiety was designed to adsorb onto the negatively charged mitochondrial membrane through electrostatic interaction. The FLW showed a large two-photon absorption (TPA) cross section (· (2) ) with 750 GM (1 GM = 10 ¹50 cm 4 s photon ¹1 molecule ¹1 ) at 730 nm. This value is larger than those of typical mitochondria-selective probes, for example, green fluorescent protein (GFP; 8 GM) 8 and rhodamine 123 (80 GM).9 Although efficient TPA compounds are usually insoluble in the aqueous medium because of their hydrophobic character, the two cationic pyridinium moieties of FLW contribute to the increase of water solubility. FLW was soluble in water at a concentration on the order of 10 ¹4 mol L ¹1 and stained cells without using any organic solvent. This advantage allowed longtime imaging of live cells for more than 24 h because toxicity due to organic solvents was not a concern. However, the fluorescence maximum of FLW was at 545 nm, which is not in the range of the tissue optical window as well as conventional TPFM probes.In this study, we aimed to develop our molecular design for a two-photon fluorescence probe that exhibits both excitation and fluorescence in the tis...