3,5-Dianilino Substituted Difluoroboron Dipyrromethene: Synthesis, Spectroscopy, Photophysics, Crystal Structure, Electrochemistry, and Quantum-Chemical Calculations

4,4-Difluoro-3,5-bis(phenylamino)-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (1), a symmetric fluorescent difluoroboron dipyrromethene dye, has been synthesized by nucleophilic substitution of 3,5-dichloro-4,4-ifluoro-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene with aniline. The solvent-dependent spectroscopic and photophysical properties have been investigated by means of UV-vis spectrophotometry and steady-state and time-resolved fluorometry and are compared to those of the nonsymmetrically substituted 5-chloro-4,4-ifluoro-3-phenylamino-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (2). A new, generalized treatment of the solvent effect, proposed by Catalan (J. Phys. Chem. B 2009, 113, 5951-5960) and based on a set of four mutually independent, empirical scales (dipolarity, polarizability, acidity, and basicity of the medium) is the most appropriate for describing the solvatochromic shifts of the UV-vis absorption and fluorescence emission of 1 and 2. In contrast to the nonsymmetric compound 2, the symmetric dye 1 has higher fluorescence quantum yields (0.45-0.86) and longer fluorescence lifetimes (3.36-4.03 ns) in all solvents studied and its emission maxima are shifted bathochromically by similar to 45 rim. Large differences are also evident in the redox electrochemistry of the two dyes, with the symmetric analogue 1 possessing a much lower oxidation potential (similar to 600 mV) than the nonsymmetric 2. The absorption bandwidths and Stokes shifts of symmetric 1 are much smaller than those for nonsymmetric 2. For both dyes, the value of the fluorescence rate constant, [k(f) = (1.7 +/- 0.3) x 10(8) s(-1)], does not depend much on the solvent tested. X-ray diffraction analysis of 1 shows that the BODIPY core possesses a planar structure. Quantum-chemical calculations support the different photophysical behavior of the symmetric derivative 1 vis-a-vis the nonsymmetric 2.