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The probe is available as cell membrane permeable methyl ester 8\u2010(4\u2010carbomethoxyphenyl)\u20104,4\u2010difluoro\u20103\u2010[2\u2010(1<jats:italic>H\u2010<\/jats:italic>imidazol\u20104\u2010yl)ethenyl]\u20101,5,7\u2010trimethyl\u20103<jats:italic>a<\/jats:italic>,4<jats:italic>a<\/jats:italic>\u2010diaza\u20104\u2010bora\u2010<jats:italic>s<\/jats:italic>\u2010indacene (<jats:bold>I<\/jats:bold>) and corresponding water\u2010soluble sodium carboxylate, sodium 8\u2010(4\u2010carboxylatophenyl)\u20104,4\u2010difluoro\u20103\u2010[2\u2010(1<jats:italic>H\u2010<\/jats:italic>imidazol\u20104\u2010yl)ethenyl]\u20101,5,7\u2010trimethyl\u20103<jats:italic>a<\/jats:italic>,4<jats:italic>a<\/jats:italic>\u2010diaza\u20104\u2010bora\u2010<jats:italic>s<\/jats:italic>\u2010indacene (<jats:bold>II<\/jats:bold>). The fluorescence quantum yield <jats:italic>\u03a6<\/jats:italic><jats:sub>f<\/jats:sub> of ester <jats:bold>I<\/jats:bold> is very high (0.8\u20131.0) in the organic solvents tested. The fluorescence lifetime (ca. 4\u2005ns) of <jats:bold>I<\/jats:bold> in organic solvents with varying polarity\/polarizability (from cyclohexane to acetonitrile) is independent of the solvent with a fluorescence rate constant <jats:italic>k<\/jats:italic><jats:sub>f<\/jats:sub> of 2.4\u00d710<jats:sup>8<\/jats:sup>\u2005s<jats:sup>\u22121<\/jats:sup>. Probe <jats:bold>I<\/jats:bold> is readily loaded in the cytosol of live cells, where its high fluorescence intensity remains nearly constant over an extended time period. Water\u2010soluble indicator <jats:bold>II<\/jats:bold> exhibits two acid\u2013base equilibria in aqueous solution, characterized by p<jats:italic>K<\/jats:italic><jats:sub>a<\/jats:sub> values of 6.0 and 12.6. The <jats:italic>\u03a6<\/jats:italic><jats:sub>f<\/jats:sub> value of <jats:bold>II<\/jats:bold> in aqueous solution is high: 0.6 for the cationic and anionic forms of the imidazole ligand, and 0.8 for neutral imidazole. On protonation\u2013deprotonation in the near\u2010neutral pH range, UV\/Vis absorption and fluorescence spectral shifts along with isosbestic and pseudo\u2010isoemissive points are observed. This dual\u2010excitation and dual\u2010emission pH indicator emits intense green\u2010yellow fluorescence at lower pH and intense orange fluorescence at higher pH. The influence of ionic strength and buffer concentration on the absorbance and steady\u2010state fluorescence of <jats:bold>II<\/jats:bold> has also been investigated. The apparent p<jats:italic>K<\/jats:italic><jats:sub>a<\/jats:sub> of the near\u2010neutral acid\u2013base equilibrium determined by spectrophotometric and fluorometric titration is nearly independent of the added buffer and salt concentration. In aqueous solution in the absence of buffer and in the pH range 5.20\u20137.45, dual exponential fluorescence decays are obtained with decay time <jats:italic>\u03c4<\/jats:italic><jats:sub>1<\/jats:sub>=4.3\u2005ns for the cationic and <jats:italic>\u03c4<\/jats:italic><jats:sub>2<\/jats:sub>=3.3\u2005ns for the neutral form of <jats:bold>II<\/jats:bold>. The excited\u2010state proton exchange of <jats:bold>II<\/jats:bold> at near\u2010neutral pH becomes reversible on addition of phosphate (H<jats:sub>2<\/jats:sub>PO<jats:sub>4<\/jats:sub><jats:sup>\u2212<\/jats:sup>\/HPO<jats:sub>4<\/jats:sub><jats:sup>2\u2212<\/jats:sup>) buffer, and a pH\u2010dependent change of the fluorescence decay times is induced. Global compartmental analysis of fluorescence decay traces collected as a function of pH and phosphate buffer concentration was used to recover values of the deactivation rate constants of the excited cationic (<jats:italic>k<\/jats:italic><jats:sub>01<\/jats:sub>=2.4\u00d710<jats:sup>8<\/jats:sup>\u2005s<jats:sup>\u22121<\/jats:sup>) and neutral (<jats:italic>k<\/jats:italic><jats:sub>02<\/jats:sub>=3.0\u00d710<jats:sup>8<\/jats:sup>\u2005s<jats:sup>\u22121<\/jats:sup>) forms of <jats:bold>II<\/jats:bold>.<\/jats:p>","DOI":"10.1002\/chem.201002280","type":"journal-article","created":{"date-parts":[[2011,8,17]],"date-time":"2011-08-17T13:37:32Z","timestamp":1313588252000},"page":"10924-10934","source":"Crossref","is-referenced-by-count":67,"title":["Rational Design, Synthesis, and Spectroscopic and Photophysical Properties of a Visible\u2010Light\u2010Excitable, Ratiometric, Fluorescent Near\u2010Neutral pH Indicator Based on BODIPY"],"prefix":"10.1002","volume":"17","author":[{"given":"No\u00ebl","family":"Boens","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Wenwu","family":"Qin","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mukulesh","family":"Baruah","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Wim M.","family":"De\u2005Borggraeve","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Aleksander","family":"Filarowski","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nick","family":"Smisdom","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Marcel","family":"Ameloot","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Luis","family":"Crovetto","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Eva M.","family":"Talavera","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jose M.","family":"Alvarez\u2010Pez","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2011,8,17]]},"reference":[{"key":"e_1_2_6_1_2","unstructured":"\u00a0"},{"key":"e_1_2_6_2_2","volume-title":"Chemosensors of Ion and Molecule Recognition","year":"1997"},{"key":"e_1_2_6_3_2","volume-title":"Molecular Fluorescence. 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