Acceptor Energy Offset Manages Ultrafast Recombination Dynamics in Donor-Acceptor Mixtures

Abstract

Abstract.We study charge recombination rates in blends of MEH-PPV and a comprehensive set of low molecular-weight fluorine-based acceptors, using visiblepump -IR-probe photoinduced absorption technique. The positions of acceptor's HOMO-LUMO levels are engineered by addition of different chemical substituents. The results clearly show the exponential scaling of the charge recombination rate with the acceptor LUMO energy. The minimal donor-acceptor LUMO energy difference required to form a CTC state is estimated as ~0.25 eV.Ground-state charge transfer complexes (CTCs) lay in heart of many photoinduced processes in physics, chemistry, and biology. The ground-state CTCs are characterized by a partial shift of electronic density from the donor to the acceptor molecule already in the ground state. Upon optical excitation, the electron is transferred to the acceptor almost instantaneously, while back recombination occurs in the finite time. The CTC photophysical properties strongly depend on the molecular structure of the donor-acceptor pair. For instance, in polymer-fullerene blends the CTC absorption is extremely weak and hardly detectable [1]. In contrast, in blends of soluble poly-paraphenylene vinylenes (MEH-PPV) with fluorene-type acceptors, the CTC formation can be readily detected visually as a color change. In the latter type of materials, the CTC band is so intense that it could enhance the solar energy harvesting in the bandgap of both the donor and acceptor [2], similarly to push-pull copolymers [3]. Furthermore, the polymer photoxidation stability is drastically increased by the presence of the CTCs [4]. As was recognized in early studies of small-molecule CTCs [5], the acceptor electron affinity is a key parameter that determines the back electron transfer rates.Here we demonstrate that the acceptor electron affinity governs the CTC recombination rate in full accord with the Markus theory [6]. Donor-acceptor CTCs formed in blends between the conjugated polymer MEH-PPV and a family of low-molecular weight fluorene acceptors was used for their known property to form stable CTCs already in solution. To unravel recombination dynamics of the photogenerated charges, ultrafast visible-pump --IR-probe photo-induced absorption (PIA) spectroscopy [7] was applied. EPJ Web of Conferences