Molecule-like CdSe Nanoclusters Passivated with Strongly Interacting Ligands: Energy Level Alignment and Photoinduced Ultrafast Charge Transfer Processes

Semiconductor nanoclusters (SCNCs) are promising electronic materials for use in solid-state device fabrication, where device efficiency is strongly controlled by charge generation and transfer from SCNCs to their surroundings. In this paper we report the excited-state dynamics of molecule-like 1.6 nm diameter CdSe SCNCs, which are passivated with the highly conjugated ligand phenyldithiocarbamate. Femtosecond transient absorption studies reveal subpicosecond hole transfer (τ ≈ 0.9 ps) from a SCNC to its ligand shell based on strong electronic interaction and hole delocalization, and subpicosecond hot electron transfer (τ ≈ 0.2 ps) to interfacial states created by charge separation. A series of control experiments were performed by varying SCNC size (1.6 nm vs 2.9 nm) and photon energy of the pump laser (388 nm vs 490 nm) as well as addition of electron quencher (benzoquinone) and hole quencher (pyridine), which rules out alternative mechanisms and confirms the critical role of energy level alignment between the SCNC and its passivating ligands. Understanding such charge carrier transfer dynamics across the SCNC–organic molecule interface is very important to various physical phenomena such as hot carrier relaxation and multiple exciton generation, which together could aid in the design of high-efficiency solar cells and photocatalysts

Photocatalysts Based on Cobalt-Chelating Conjugated Polymers for Hydrogen Evolution from Water

Developing photocatalytic systems for water splitting to generate oxygen and hydrogen is one of the biggest chemical challenges in solar energy utilization. In this work, we report the first example of heterogeneous photocatalysts for hydrogen evolution based on in-chain cobalt-chelating conjugated polymers. Two conjugated polymers chelated with earth-abundant cobalt ions were synthesized and found to evolve hydrogen photocatalytically from water. These polymers are designed to combine functions of the conjugated backbone as a light-harvesting antenna and electron-transfer conduit with the in-chain bipyridyl-chelated transition metal centers as catalytic active sites. In addition, these polymers are soluble in organic solvents, enabling effective interactions with the substrates as well as detailed characterization. We also found a polymer-dependent optimal cobalt chelating concentration at which the highest photocatalytic hydrogen production (PHP) activity can be achieved

Molecular Structure Controlled Transitions between Free-Charge Generation and Trap Formation in a Conjugated Copolymer Series

Rational design strategies for controlling the energetics of conjugated “donor–acceptor” copolymers are ubiquitous in the literature, as they allow for simple energy-level tuning strategies to be employed for photovoltaic and transistor applications. Utilizing the recently reported <b>PTR<i>n</i></b> series of conjugated polymers closely related to the widely implemented material <b>PTB7</b>, we investigate the effect of local copolymer block energetics on the generation of transient excitonic and charge carrier species. It is clearly demonstrated that local copolymer block energetics play a much larger role than is apparent from simple energy-level tuning arguments, and drastically affect the ultrafast generation of free-charge carrier and trap state populations. Specifically, we observe an almost complete reversal in the efficient generation of free-charge in <b>PTB7</b> to the ultrafast creation of a high percentage of trapped pseudo charge-transfer states. The implications of this secondary effect of “donor–acceptor” energy level tuning are discussed, along with strategies for avoiding the generation of trap states in “donor–acceptor” copolymers