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

## TL;DR

A new BODIPY derivative with a twisted anthracene structure shows unique photophysical properties, including solvent-dependent fluorescence and a long-lived triplet state.

## Contribution

The study reveals how molecular torsion and solvent polarity influence intersystem crossing and non-radiative relaxation in a twisted BODIPY derivative.

## Key findings

- The fluorescence quantum yield of AN-BDP decreases from 59% in toluene to 3% in acetonitrile.
- The triplet state of AN-BDP has a long lifetime of 551 μs and a moderate intersystem crossing yield of 10%.
- Theoretical calculations suggest a solvent-dependent energy barrier for conical intersections affecting fluorescence.

## Abstract

The photophysical properties of a BODIPY derivative with the highly twisted molecular structure of anthracene-fused boron–dipyrromethene (AN-BDP) were studied with steady-state and time-resolved spectroscopic methods. The fused anthryl and the BDP units in AN-BDP 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 D and E tensors as (−71.4 mT, 16.7 mT, respectively) compared to the triplet state of the iodinated native BDP (D = −104.6 mT, E = 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.