Photophysical and Electrochemical Characterization of BODIPY-Containing Dyads Comparing the Influence of an A-D-A versus D-A Motif on Excited-State Photophysics.

Abstract

A complete photophysical characterization of organic molecules designed for use as molecular materials is critical in the design and construction of devices such as organic photovoltaics (OPV). The nature of a molecule's excited state will be altered in molecules employing the same chromophoric units but possessing different molecular architectures. For this reason, we examine the photophysical reactions of two BODIPY-based D-A and A-D-A molecules, where D is the donor and A is the acceptor. A BODIPY (4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene) moiety serves as the A component and is connected through the meso position using a 3-hexylthiophene linker to a N-(2-ethylhexyl)dithieno[3,2-b:2',3'-d]pyrrole (DTP), which serves as the D component. An A-D-A motif is compared to its corresponding D-A dyad counterpart. We show a potential advantage to the A-D-A motif over the D-A motif in creating longer-lived excited states. Transient absorption (TA) spectroscopy is used to characterize the photophysical evolution of each molecule's excited state. Global analysis of TA data using singular value decomposition and target analysis is performed to identify decay-associated difference spectra (DADS). The DADS reveal the spectral features associated with charge-transfer excited states that evolve with different dynamics. A-D-A possess slightly longer excited-state lifetimes, 42 ps nonradiative decay, and 4.64 ns radiative decay compared to those of D-A, 24 ps nonradiative decay, and 3.95 ns radiative decay. A longer lived A-D-A component is observed with microsecond lifetimes, representing a small fraction of the total photophyscial product. Steady-state and time-resolved photoluminescence augment the insights from TA, while electrochemistry and spectroelectrochemistry are employed to identify the nature of the excited state. Density functional theory supports the observed electronic and electrochemical properties of the D-A and A-D-A molecules. These results form a complete picture of the electronic and photophysical properties of D-A and A-D-A and provide contextualization for structure-function relationships between molecules and OPV devices.