Generalized solvent scales as a tool for investigating solvent dependence of spectroscopic and kinetic parameters. Application to fluorescent BODIPY dyes

Abstract

Two difluoroboron dipyrromethene (BODIPY) based fluorescent dyes - 4,4-difluoro-3-{2-[4-(dimethylamino)phenyl]ethenyl}-8-[4-(methoxycarbonyl)phenyl]-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene (1) and 4,4-difluoro-3-[2-(4-fluoro-3-hydroxyphenyl)ethenyl]-8-[4-(methoxycarbonyl)phenyl]-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene (3) - have been synthesized via condensation of p-N,N-dimethylaminobenzaldehyde and 4-fluoro-3-hydroxybenzaldehyde, respectively, with 4,4-difluoro-8-[4-(methoxycarbonyl)phenyl]-1,3,5,7-tetramethyl-3a,4a-diaza-4-bora-s-indacene (2). UV-vis spectrophotometry and steady-state and time-resolved fluorometry have been used to study the spectroscopic and photophysical characteristics of in various solvents. The multi-parameter Kamlet-Taft {pi*, alpha, beta} solvent scales and a new, generalized treatment of the solvent effect, proposed by Catalán (J. Phys. Chem. B, 2009, 113, 5951-5960), have been used in the analysis of the solvatochromic shifts of the UV-vis absorption and fluorescence emission maxima of 1-3, and the rate constants of excited-state deactivation via fluorescence (k(f)) and radiationless decay (k(nr)). The four Catalán solvent scales (dipolarity, polarizability, acidity and basicity of the medium) are the most appropriate for describing the solvatochromic effects. Solvent dipolarity and polarizability are the important causes for the solvatochromism of 1. Conversely, the absorption and emission maxima of 2 and 3 are hardly dependent on the solvent: the small changes reflect primarily the polarizability of the solvent surrounding the dye. Fluorescence decay profiles of 1 can be described by a single-exponential function in aprotic solvents, whereas two decay times are found in alcohols. The fluorescence decays of 2 (lifetimes tau in 1.9-2.9 ns range) and 3 (tau between 3.5 and 4.0 ns) are mono-exponential in all solvents studied. The fluorescence properties of dye are very sensitive to the solvent: upon increasing solvent dipolarity, the fluorescence quantum yields and k(f) values decrease and the emission maxima become more red-shifted. The k(f) values of 2 [(1.6 +/- 0.3) x 10(8) s(-1)] and 3 [(1.5 +/- 0.2) x 10(8) s(-1)] are practically independent of the solvent properties. The crystal structure of reveals that the BODIPY core is nearly planar with the boron atom moved out of the plane. The angle between the phenyl group at the meso-position and the BODIPY plane equals 80 degrees.

High Stokes Shift Anilido‐Pyridine Boron Difluoride Dyes

“…In contrast to the majority of BODIPY dyes, [18] the photophysical properties of which are relatively insensitive to solvent polarity, the anilido-pyridine supported dyes show moderate hypsochromic shifts of the S 0 !S 1 transition in more polar solvents. For example, 3 b gives a l max of 452 nm in hexane, but 426 nm in DMSO, suggesting that the excited state is less polar than the ground state in these molecules.…”

Effect of the substitution position (2, 3 or 8) on the spectroscopic and photophysical properties of BODIPY dyes with a phenyl, styryl or phenylethynyl group

“…In a pioneering paper, we analyzed the solvent-dependent behavior of the kinetic parameters kf and knr of three boron dipyrromethene dyes. 44 Since kf (= /) and knr [(1 -)/] are calculated from the values of fluorescence quantum yield () and lifetime (), obtaining accurate values of kf and knr depends critically on the accuracy with which  and  are measured. These measurements are generally more challenging than those of abs and em.…”

Incorporating a Piperidinyl Group in the Fluorophore Extends the Fluorescence Lifetime of Click-Derived Cyclam-Naphthalimide Conjugates

“…alcohols) are typically greater than those in solvents that less readily form hydrogen bonds ( e.g. toluene); such behavior can be attributed to protic solvent-fluorophore hydrogen bonding and has been observed for other fluorophores [42], [43].…”