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Fluorinated Boron-Dipyrromethene (BODIPY) dyes: bright and versatile probes for surface analysis
(2013)
A family of bright boron-dipyrromethene-type fluorophores with a high number of fluorine atoms (F-BODIPYs) has been developed and characterized by X-ray crystallography and optical spectroscopy. The introduction of 3,5-bis(trifluoromethyl)phenyl and pentafluorophenyl moieties significantly enhances the photostability of such dyes, yielding for instance photostable near-infrared (NIR) fluorophores that show emission maxima>750 nm, when the BODIPY's π system is extended with two (dimethylamino)styryl and (dimethylamino)naphthastyryl moieties, or green-emitting BODIPYs with fluorescence quantum yields of unity. When equipped with a suitable group that selectively reacts for instance with amines, F-BODIPYs can be used as potent dual labels for the quantification of primary amino groups on surfaces by X-ray photoelectron spectroscopy (XPS) and fluorescence, two powerful yet complementary tools for the analysis of organic surface functional groups. The advantage of reactive F-BODIPYs is that they allow a fast and non-destructive mapping of the labelled supports with conventional fluorescence scanners and a subsequent quantification of selected areas of the same sample by the potentially traceable XPS technique. The performance is exemplarily shown here for the assessment of the amino group density on SiO2 supports, one of the most common reactive silica supports, in particular, for standard microarray applications.
We present a comparative study of the spectroscopic properties of the donor–acceptor–donor substituted dyes triphenylamine-allylidenemalononitrile-julolidine (TMJ) and triphenylamine-allylidenemalononitriletriphenylamine (TMT), bearing one and two propeller-like triphenylamine donor moieties, in solvents of varying polarity and viscosity and in the aggregated and solid state. Our results reveal control of the aggregation-induced spectroscopic changes and the packing motifs of the dye molecules in the solid state by the chemical nature and structure of the second nitrogen-containing donor, i.e., a planar and a rigid julolidine or a twisted triphenyl group. Assuming that the TMT and TMJ aggregates show a comparable arrangement of the molecules to the respective crystals, these different molecular interactions in the solid state are responsible for aggregation induced emission (AIE) in the case of TMT and its absence for TMJ. Moreover, a versatile strategy for the fluorescence enhancement of only weakly emissive AIE dyes is shown, turning these dyes into bright nanoscale fluorescent reporters by using them as stains for preformed polymer particles.
We report on the temperature- and structural-dependent optical properties and photophysics of a set of boron dipyrromethene (BODIPY) dyes with different substitution patterns of their meso-aryl subunit. Single-crystal Xray diffraction analysis of the compounds enabled a classification of the dyes into a sterically hindered and a unhindered group. The steric hindrance refers to a blocked rotational motion of the aryl subunit around the bond connecting this moiety to the meso-position of the BODIPY core. The energy barriers related to this rotation were simulated by DFT calculations. As follows from the relatively low rotational barrier calculated to about 17 kcal/mol, a free rotation is only possible for sterically unhindered compounds. Rotational barriers of more than 40 kcal/mol determined for the sterically hindered compounds suggest an effective freezing of the rotational motion in These molecules. With the aid of temperature-dependent spectroscopic measurements, we could show that the ability to rotate directly affects the optical properties of our set of BODIPY dyes. This accounts for the strong temperature dependence of the fluorescence of the sterically unhindered compounds which show a drastic decrease in fluorescence quantum yield and a significant shortening in fluorescence lifetime upon heating. The optical properties of the sterically hindered compounds, however, are barely affected by temperature. Our results suggest a nonradiative deactivation of the first excited singlet state of the sterically unhindered compounds caused by a conical intersection of the potential energy surfaces of the Ground and first excited state which is accessible by rotation of the meso-subunit. This is in good agreement with previously reported deactivation mechanisms. In addition, our results suggest the presence of a second nonradiative depopulation pathway of the first excited singlet state which is particularly relevant for the sterically hindered compounds.