# Anticipating acene-based chromophore spectra with molecular orbital arguments

## TL;DR

This study combines synthesis, electronic structure theory, and spectral analysis to explain and predict the absorption spectra of acene-based chromophores, revealing a design principle for creating highly absorbent molecules.

## Contribution

It introduces a molecular orbital-based rule to predict absorption features in acene dimers, explaining a long-standing spectral phenomenon and guiding chromophore design.

## Key findings

- Charge-transfer excitations borrow intensity from UV states.
- The bonding geometry determines the presence of additional absorption.
- The rule accurately predicts spectra of various acene derivatives.

## Abstract

Recent synthetic studies on the organic molecules tetracene and pentacene have found certain dimers and oligomers to exhibit an intense absorption in the visible region of the spectrum which is not present in the monomer or many previously-studied dimers. In this article we combine experimental synthesis with electronic structure theory and spectral computation to show that this absorption arises from an otherwise dark charge-transfer excitation 'borrowing intensity' from an intense UV excitation. Further, by characterizing the role of relevant monomer molecular orbitals, we arrive at a design principle that allows us to predict the presence or absence of an additional absorption based on the bonding geometry of the dimer. We find this rule correctly explains the spectra of a wide range of acene derivatives and solves an unexplained structure-spectrum phenomenon first observed seventy years ago. These results pave the way for the design of highly absorbent chromophores with applications ranging from photovoltaics to liquid crystals.