BODIPY dyes and their derivatives: syntheses and spectroscopic properties.

The world of organic luminophores has been confined for a long time to fairly standard biological labeling applications and to certain analytical tests. Recently, however, the field has undergone a major change of direction, driven by the dual needs to develop novel organic electronic materials and to fuel the rapidly emerging nanotechnologies. Among the many diverse fluorescent molecules, the Bodipy family, first developed as luminescent tags and laser dyes, has become a cornerstone for these new applications. The near future looks extremely bright for "porphyrin's little sister".

The Chemistry of Fluorescent Bodipy Dyes: Versatility Unsurpassed

In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin. In the intervening years, the fundamental binding properties of CB[6]-high affinity, highly selective, and constrictive binding interactions--have been delineated by the pioneering work of the research groups of Mock, Kim, and Buschmann, and has led to their applications in waste-water remediation, as artificial enzymes, and as molecular switches. More recently, the cucurbit[n]uril family has grown to include homologues (CB[5]-CB[10]), derivatives, congeners, and analogues whose sizes span and exceed the range available with the alpha-, beta-, and gamma-cyclodextrins. Their shapes, solubility, and chemical functionality may now be tailored by synthetic chemistry to play a central role in molecular recognition, self-assembly, and nanotechnology. This Review focuses on the synthesis, recognition properties, and applications of these unique macrocycles.

The cucurbit[n]uril family

This critical review covers the advances made using the 4-bora-3a,4a-diaza-s-indacene (BODIPY) scaffold as a fluorophore in the design, synthesis and application of fluorescent indicators for pH, metal ions, anions, biomolecules, reactive oxygen species, reactive nitrogen species, redox potential, chemical reactions and various physical phenomena. The sections of the review describing the criteria for rational design of fluorescent indicators and the mathematical expressions for analyzing spectrophotometric and fluorometric titrations are applicable to all fluorescent probes (206 references).

Fluorescent indicators based on BODIPY

Rare-earth upconversion nanophosphors (UCNPs), when excited by continuous-wave near-infrared light, exhibit a unique narrow photoluminescence with higher energy. Such special upconversion luminescence makes UCNPs promising as bioimaging probes with attractive features, such as no auto-fluorescence from biological samples and a large penetration depth. As a result, UCNPs have emerged as novel imaging agents for small animals. In this critical review, recent reports regarding the synthesis of water-soluble UCNPs and their surface modification and bioconjugation chemistry are summarized. The applications of UCNPs for small-animal imaging, including tumor-targeted imaging, lymphatic imaging, vascular imaging and cell tracking are reviewed in detail. The exploration of UCNPs as multifunctional nanoscale carriers for integrated imaging and therapy is also presented. The biodistribution and toxicology of UCNPs are further described. Finally, we discuss the challenges and opportunities in the development of UCNP-based nanoplatforms for small-animal imaging (276 references).

Upconversion nanophosphors for small-animal imaging

Four novel borondipyrromethene (BDP) and -diindomethene (BDI) dyes with one or two (dimethylamino)styryl extensions at the chromophore were synthesized and spectroscopically investigated. An X-ray crystal structure shows that the extended auxochrome is virtually planar. All dyes thus display intense red/near infrared (NIR) absorption and emission. The (dimethylamino)styryl group induces a charge-transfer character that entails bright solvatochromic fluorescence, which is only quenched with increasing solvent polarity according to the energy-gap law. The dye with an additional dimethylanilino group at the meso position of BDP shows a remarkable switching of lipophilicity by protonation. Two dyes with an 8-hydroxyquinoline ligand at the meso position display quenched emission in the presence of Hg2+ or Al3+ owing to electron transfer from the excited BDP to the complexed receptor. The BDI dye presents a pH indicator with bright fluorescence and extremely low fluorescence anisotropy.

Mono- and di(dimethylamino)styryl-substituted borondipyrromethene and borondiindomethene dyes with intense near-infrared fluorescence.

The methylene-bridged triphenylamine 2 has been oxidized to planar radical cation 2•+ by B(C6F5)3 or Ag+. Further reaction of 2•+[Al(ORF)4]− and 2 with trace amounts of silver salt resulted in dication 32+, providing a rare example of structurally characterized bis(triarylamine) “bipolarons”. 32+ can be directly prepared by the reaction of 3 with 2 equiv of Ag+. X-ray structural analysis together with theoretical calculation shows that 32+ has singlet diradical character and is analogous to Chichibabin’s hydrocarbons.

One-Electron Oxidation of an Organic Molecule by B(C6F5)3; Isolation and Structures of Stable Non-para-substituted Triarylamine Cation Radical and Bis(triarylamine) Dication Diradicaloid

Salts containing triarylphosphine radical cations 1•+ and 2•+ have been isolated and characterized by electron paramagnetic resonance (EPR) and UV–vis absorption spectroscopy as well as single-crystal X-ray diffraction. Radical 1•+ exhibits a relaxed pyramidal geometry, while radical 2•+ becomes fully planar. EPR studies and theoretical calculations showed that the introduction of bulky aryl groups leads to enhanced p character of the singly occupied molecular orbital, and the radicals become less pyramidalized or fully flattened.

Isolation and X-ray Crystal Structures of Triarylphosphine Radical Cations

Iodine-assisted assembly of helical coordination polymers of cucurbituril and asymmetric copper(II) complexes

Salts containing tetraaryldiphosphine radical cation 1•+ and dication 12+ have been isolated and structurally characterized. Radical 1•+ has a relaxed pyramidal geometry, while dication 12+ prefers a planar, olefin-like geometry with a two-electron π bond. The alteration of the geometries of the tetraaryldiphosphine upon oxidation is rationalized by the nature of the bonding. The EPR spectrum showed that the spin density of radical 1•+ is mainly localized on phosphorus atoms, which is supported by theoretical calculation.

Yi-Zhi Li

Papers

Mono- and di(dimethylamino)styryl-substituted borondipyrromethene and borondiindomethene dyes with intense near-infrared fluorescence.

One-Electron Oxidation of an Organic Molecule by B(C6F5)3; Isolation and Structures of Stable Non-para-substituted Triarylamine Cation Radical and Bis(triarylamine) Dication Diradicaloid

Isolation and X-ray Crystal Structures of Triarylphosphine Radical Cations

Iodine-assisted assembly of helical coordination polymers of cucurbituril and asymmetric copper(II) complexes

Stable tetraaryldiphosphine radical cation and dication.