# New Insights into Intersystem Crossing in Substituted Aromatics: Singlet–Triplet Conversion in Carbonyl-Substituted Anthracenes

## TL;DR

This study explores how the orientation of carbonyl groups affects intersystem crossing in aromatic molecules, revealing how substituent geometry influences fast energy conversion processes.

## Contribution

The study provides new insights into how substituent reorientation and slanted geometries accelerate intersystem crossing in carbonyl-substituted aromatics.

## Key findings

- In 9-acetylanthracene, intersystem crossing occurs after the carbonyl group adopts a slanted geometry.
- Manifold crossing in 9AA happens on a 3 to 25 ps time-scale, depending on the solvent.
- The first excited singlet in 9AA retains ππ* character and decays through ISC without other singlet states.

## Abstract

A new study is presented
to elucidate the photodynamics of model
carbonyl-substituted polyaromatics targeting the relevance of carbonyl-group
orientation and torsional re-equilibration on intersystem crossing
(ISC). Our experiments focused on 9-acetylanthracene (9AA) using femtosecond
resolved spectroscopy. In the ground state of this molecule, steric
interactions force the carbonyl substituent into a near-perpendicular
orientation relative to the aromatic system. The time-resolved signals
from 9AA show that ISC takes place after spectral shifts that reflect
the evolution of the carbonyl group to a slanted geometry as it adjusts
to a dihedral angle of around 40° with respect to the aromatic
plane. Depending on the solvent, in 9AA manifold crossing takes place
on the 3 to 25 ps time-scale. On the other hand, for 2-acetylanthracene
(2AA) which is coplanar in both S0 and S1, the
emission lifetimes can reach several nanoseconds. Analysis of these
systems at the highest available theoretical levels reveals further
insights into the excited-state dynamics. For 9AA and in contrast
with previous publications, it is established that for all relevant
geometries, the first excited singlet retains a ππ* character
and decays through ISC with no involvement of other singlet states.
The manifold crossing involves the interaction with the triplet manifold
through states which’s transition orbitals are partially localized
at the acetyl substituent. Specifically, the slanted geometry of the
carbonyl group in 9AA and the potential energy surface around the
equilibrium S1 geometry implies significant spin–orbit
interactions and accelerated manifold-crossings. The present results
highlight the relevance of substituent reorientation and their slanted
geometries which appear to be a dominant feature in carbonyl and nitrated
aromatic systems which show rapid ISC dynamics. In the article, we
include details on the differences in the mechanisms operating in
these two kinds of systems which show the fastest ISC rates among
organic chromophores.