Photoinduced Electron Transfer and Changes in Surface Free Energy in Polythiophene-Polyviologen Bilayered Thin Films

Abstract

Bipyridiniums, also known as viologens, are well-documented electron acceptors that are generally easy to synthesize on a large scale and reversibly cycle between three oxidation states (V2+, V•+, and V0). Accordingly, they have been explored in a number of applications that capitalize on their dynamic redox chemistry, such as redox-flow batteries and electrochromic devices. Viologens are also particularly useful in photoinduced electron transfer (PET) processes and therefore are of interest in photovoltaic applications that typically rely on electron-rich donors like polythiophene (PTh). However, the PET mechanism and relaxation dynamics between interfacing PTh and viologen-based thin films has not been well studied as a function of thickness of the acceptor layer. Here, a novel, bilayered thin film composite was fabricated by first spin-coating PTh onto glass slides, followed by spin-coating and curing polyviologen (PV)-based micron-sized films of variable thicknesses (0.5–11.3 μm) on top of the PTh layer. The electron-transfer mechanism and relaxation dynamics from the PTh sublayer into the upper PV film were investigated using femtosecond transient absorption (fTA) spectroscopy and electrochemistry to better understand how the charge-transfer/relaxation lifetimes could be extended using thicker PV acceptor films. The fTA experiments were performed under inert N2 conditions as well as in ambient O2. The latter shortened the lifetimes of the electrons in the PV layer, presumably due to O2 triplet-based trap sites. Contact angle measurements using H2O and MeI were also performed on top of the bilayered films to measure changes in surface free energy that would aid the assessment related to efficiency of the combined processes involving light penetration, photoexcitation, electron mobility, and relaxation from within the bilayered thin films. Insights gained from this work will support the development of future devices that employ viologen-based materials as an alternative electron-acceptor that is both easily processable and scalable.

Ultrafast Dynamics of Photoinduced Electron Transfer in Bay-Aryl-Substituted Perylene Diimide Derivatives

“…Understanding PET in the active materials for organic electronics, mainly for photovoltaic applications, is crucial for better device efficiency. − For solar cells, electron transfer followed by exciton splitting is desired to collect the charge carriers at the respective electrodes. Donor–acceptor systems having suitable redox potentials have shown photoinduced intramolecular or intermolecular electron transfer. − From these studies it has been found that PET is affected by several factors such as (i) the electronic structures of the donor and acceptor, (ii) the distance between them, and (iii) environmental effects such as the solvent in solution and the morphology in the solid state. − …”

“…Photoexcited electron transfer in organic dyes and transition metal complexes has been studied by several research groups because of their photophysical properties. − In these systems, electron transfer either from the organic dye or from the metal center in both directions has been studied. − Different families of organic dyes, including perylene diimides (PDIs), have been studied to explore PET. PDIs offer a high absorption coefficient, photostability, and ease of synthetic manipulation, making them suitable for various applications involving electron transfer. − PDI derivatives attached to a heteroleptic iridium complex were used to show single-electron oxidation of a catalyst precursor for water oxidation, and a D–A system involving PDI was demonstrated to show excellent power conversion efficiency in organic solar cells .…”

Organogel Actuators Based on Redox-Responsive Foldamers: Amplification of Molecular Machine’s Motion to Macroscopic Scale

“…25,26 Since a volume change in the gel occurred during the reduction process, the actuator could work only under anaerobic conditions inside a N 2 -filled glovebox. 25,27,28 This is a serious drawback for practical application. Based on this perspective, we aim to develop the foldamerbased gel actuators that also function under aerobic conditions (under oxidative conditions in air).…”

“… − Barnes et al reported on the redox-responsive actuation of the hydrogel consisting of linearly connected viologen polymers . When the viologen unit was reduced to the radical cation state, the conformation of the viologen polymer was found to alter from the extended state to the folded state due to the radical-based self-assembly of the viologen units. , Since a volume change in the gel occurred during the reduction process, the actuator could work only under anaerobic conditions inside a N 2 -filled glovebox. ,, This is a serious drawback for practical application.…”