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-difluoro-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-difluoro-3-phenylamino-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (2). A new, generalized treatment of the solvent effect, proposed by Cataln (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 45 nm. Large differences are also evident in the redox electrochemistry of the two dyes, with the symmetric analogue 1 possessing a much lower oxidation potential (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, [kf = (1.7 ± 0.3) × 108 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--vis the nonsymmetric 2.