| Abstract
| - The photophysical properties of seven new 8-(p-substituted)phenyl analogues of 4,4-difluoro-3,5-dimethyl-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (derivatives of the well-known fluorophore BODIPY) in several solventshave been studied by means of absorption and steady-state and time-resolved fluorimetry. For each compound,the fluorescence quantum yield and lifetime are lower in solvents with higher polarity owing to an increasein the rate of nonradiative deactivation. Increasing the electron withdrawing strength of the p-substituent onthe phenyl group in position 8 also leads to lower fluorescence quantum yields and lifetimes. When thep-substituent on the phenyl group in position 8 is a tertiary amine [8-(4-piperidinophenyl), 8-(4-N,N-dimethylaminophenyl), and 8-(4-morpholinophenyl)], the low quantum yields of these compounds in morepolar solvents can be rationalized by the inversion of the energy levels of an apolar, highly fluorescent anda polar, nonfluorescent excited state, where charge transfer from the tertiary amine to the BODIPY unitoccurs. These amine analogues can be protonated at low pH in aqueous solution. Fluorescence titrationsyielded pKa values of their conjugate ammonium salts which are in agreement with the electron donatingtendency of the amine group: piperidino (4.15) > dimethylamino (2.37) > morpholino (1.47), with the pKavalues in parentheses. The rate constant of radiative deactivation (kf) is the same for all compounds in allsolvents studied (kf = 1.4 × 108 s-1).
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