# Tetracene Diacid Aggregates for Directing Energy Flow toward Triplet Pairs

## TL;DR

This study explores how tetracene derivatives form aggregates that influence energy transfer, offering new insights for light-harvesting applications.

## Contribution

The novel contribution is the discovery of how hydrogen bonding and aggregation structures affect energy flow in tetracene derivatives.

## Key findings

- Tetracene diacid aggregates form excimers and mixed states with charge-transfer and triplet pair character.
- Cofacial dimer structures are the most stable and influence absorption and energy transfer.
- Hydrogen bonding in Tc-DA promotes excimer formation, unlike in the non-hydrogen-bonding Tc-DE.

## Abstract

A comprehensive investigation
of the solution-phase photophysics
of tetracene bis-carboxylic acid [5,12-tetracenepropiolic acid (Tc-DA)] and its related methyl ester [5,12-tetracenepropynoate (Tc-DE)], a non-hydrogen-bonding counterpart, reveals
the role of the carboxylic acid moiety in driving molecular aggregation
and concomitant excited-state behavior. Low-concentration solutions
of Tc-DA exhibit similar properties to the popular 5,12-bis((triisopropylsilyl)ethynl)tetracene,
but as the concentration increases, evidence for aggregates that form
excimers and a new mixed-state species with charge-transfer (CT) and
correlated triplet pair (TT) character is revealed by transient absorption
and fluorescence experiments. Aggregates of Tc-DA evolve
further with concentration toward an additional phase that is dominated
by the mixed CT/TT state which is the only state present in Tc-DE aggregates and can be modulated with the solvent polarity.
Computational modeling finds that cofacial arrangement of Tc-DA and Tc-DE subunits is the most stable aggregate structure
and this agrees with results from 1H NMR
spectroscopy. The calculated spectra of these cofacial dimers
replicate the observed broadening in ground-state absorption as well
as accurately predict the formation of a near-UV transition associated
with a CT between molecular subunits that is unique to the specific
aggregate structure. Taken together, the results suggest that the
hydrogen bonding between Tc-DA molecules and the associated
disruption of hydrogen bonding with solvent produce a regime of dimer-like
behavior, absent in Tc-DE, that favors excimers rather
than CT/TT mixed states. The control of aggregate size and structure
using distinct functional groups, solute concentration, and solvent
in tetracene promises new avenues for its use in light-harvesting
schemes.