Probing the electronic structure and photophysics of thiophene-diketopyrrolopyrrole derivatives in solution

TL;DR

This study investigates the electronic structure and photophysical behavior of thiophene-diketopyrrolopyrrole derivatives using ultrafast spectroscopy, revealing how electronic states influence optical properties and fluorescence efficiency.

Contribution

It provides new insights into the excited-state dynamics of TDPP molecules, linking electronic structure to optical properties through ultrafast transient absorption analysis.

Findings

Photophysics governed by two distinct excited states with different electronic characters.

Fluorescence quantum yields vary by 1-2 orders of magnitude among TDPP prototypes.

Non-radiative transfer mediated by conical intersections influences excited-state dynamics.

Abstract

Diketopyrrolopyrroles are a popular class of electron-withdrawing unit in optoelectronic materials. When combined with electron donating side-chain functional groups such as thiophenes, they form a very broad class of donor-acceptor molecules: thiophene-diketopyrrolopyrroles (TDPPs). Despite their widescale use in biosensors and photovoltaic materials, studies have yet to establish the important link between the electronic structure of the specific TDPP and the critical optical properties. To bridge this gap, ultrafast transient absorption with 22 fs time resolution has been used to explore the photophysics of three prototypical TDPP molecules: a monomer, dimer and polymer in solution. These studies show that the photophysics of these molecular prototypes under visible photoexcitation are determined by just two excited electronic states, having very different electronic characters (one is optically bright, the other dark), their relative energetic ordering and the timescales for internal conversion from one to the other and/or to the ground state. The underlying difference in electronic structure alters the branching between these excited states and their associated dynamics. In turn, these factors dictate the fluorescence quantum yields, which are shown to vary by ~1-2 orders of magnitude across the TDPP prototypes investigated here. The fast non-radiative transfer of molecules from the bright to dark states is mediated by conical intersections. Remarkably, wavepacket signals in the measured transient absorption data carry signatures of the nuclear motions that enable mixing of the electronic-nuclear wavefunction and facilitate non-adiabatic coupling between the bright and dark states.