Photophysical and laser emission studies of 8-polyphenylene-substituted BODIPY dyes in liquid solution and in solid polymeric matrices

In our search for efficient and photostable laser dyes, four new dyes with the basic structure of the commercial BODIPY laser dye PM567, with either an 8-diphenylene or an 8-p-triphenylene group, both substituted at the terminal polyphenylene position with an acetoxymethyl (dyes P2Ar1Ac and P3Ar1Ac, respectively) or a methacryloyloxymethyl group (dyes P2Ar1MA and P3Ar1MA, respectively), have been synthesized. The photophysical and lasing properties of the dyes have been studied both dissolved in liquid solvents (acetoxymethyl dyes) and incorporated into solid polymeric matrices, in the latter case as solutions (acetoxymethyl dyes) or as copolymers with methyl methacrylate (methacryloyloxymethyl dyes). In liquid solution, the photophysics of P2Ar1Ac and P3Ar1Ac is scarcely affected by the number (two or three) of p-phenylene units. Quantum mechanical calculations reveal that the p-phenylene units in these dyes are twisted ca. 37 degrees each other, an that the first 8-p-phenylene group stands nearly perpendicular to the aromatic BODIPY plane, resulting in electronic decoupling of the two chromophores. P2Ar1Ac exhibits a somewhat lower photodegradation quantum yield under UV and visible irradiation, if compared with P3Ar1Ac or with PM567, likely because of its also lower rate constant for the reaction with in situ-generated singlet molecular oxygen. Both acetoxymethyl dyes emit laser radiation in solution in all the solvents tried, under transversal pumping at 532 nm. In ethyl acetate, with a dye concentration of 0.80 x 10(-3) M, laser efficiencies as high as 80% have been observed. When the 8-polyphenylene dyes were incorporated into solid poly(methyl methacrylate) (PMMA) matrices, as solutions or as copolymers, the fluorescence emission increased with respect to that of the parent PM567 dye dissolved in the same matrix, and lasing efficiencies in the range 18-31% were obtained, with good photostability. The dye P2Ar1Ac dissolved in PMMA was found to exhibit the best overall laser behavior, with a good balance between efficiency and photostability.