Synthesis, spectral properties and photostability of novel boron–dipyrromethene dyes

Two series of novel boron–dipyrromethene (BODIPY) dyes containing 8-phenyl groups have been synthesized and their spectral properties have been studied. Series 2 with four methyl groups at 1,3,5,7-positions show much higher fluorescence quantum yields and extinction coefficients than series 1 (without methyl groups). The “push–pull” electronic effect caused by the methyl groups at 3 and 5 positions is a significant positive factor to the high quantum yields of 2. The X-ray structure analysis of 1c and 2c reveals that steric expulsion exists between the phenyl and adjacent two methyl groups. Moreover, the steric expulsion might block the intramolecular vibronic relaxation and internal conversion of the excited 2, which also contributes to their high fluorescence quantum yields. The substituent effects on photostability and redox potentials of these dyes have been discussed.