Twisted Anthracene-Fused BODIPY: Intersystem Crossing and Torsion-Induced Non-Radiative Relaxation of the Singlet Excited State

The photophysical properties of a BODIPY derivative with the highly twisted molecular structure of anthracene-fused boron-dipyrromethene (<b>AN-BDP</b>) were studied with steady-state and time-resolved spectroscopic methods. The fused anthryl and the BDP units in <b>AN-BDP</b> units both adopt distorted geometry (with ca. 10° of torsion), and there is large dihedral angle between the two units (ca. 49.7°). Interestingly, the fluorescence quantum yields are highly dependent on the solvent polarity (59~3%, from toluene to acetonitrile), yet the fluorescence emission wavelength does not change in different solvents. Nanosecond transient absorption spectra indicate that the triplet state is long-lived, with an intrinsic triplet state lifetime of 551 μs. Interestingly the severely twisted structure only shows a moderate intersystem crossing (ISC) yield (10%). Femtosecond transient absorption spectra indicate slow ISC (>1.5 ns), which is in agreement with the fluorescence lifetime (2.3 ns). Time-resolved electron paramagnetic resonance (TREPR) spectra show smaller zero-field-splitting <i>D</i> and <i>E</i> tensors as (-71.4 mT, 16.7 mT, respectively) compared to the triplet state of the iodinated native BDP (<i>D</i> = -104.6 mT, <i>E</i> = 22.8 mT), inferring that the triplet-state wave function of the new compound is delocalized over the twisted molecular framework. The theoretical computation indicated a solvent-polarity-dependent energy barrier for the relaxed S₁ state to a conical interaction (CI) of the S₁ and the S₀ state potential curves, which agrees with the weaker fluorescence in polar solvents.