Mixed Organic Ligand Shells: Controlling the Nanoparticle Surface Morphology toward Tuning the Optoelectronic Properties

Precise control over the ratio of perylene bisimide (PBI) monomers and aggregates, immobilized on alumina nanoparticle (NP) surfaces, is demonstrated. Towards this goal, phosphonic acid functionalized PBI derivatives (PA-PBI) are shown to self-assemble into stoichiometrically mixed monolayers featuring aliphatic, glycolic, or fluorinated phosphonic acid ligands, serving as imbedding matrix (PA-M) to afford core-shell NPs. Different but, nevertheless, defined PBI monomer/aggregate composition is achieved by either the variation in the PA-PBI to PA-M ratios, or the utilization of different PA-Ms. Various steady-state as well as time-resolved spectroscopy techniques are applied to probe the core-shell NPs with respect to changes in their optical properties upon variations in the shell composition. To this end, the ratio between monomer and excimer-like emission assists in deriving information on the self-assembled monolayer composition, local ordering, and corresponding aggregate content. With the help of X-ray reflectivity measurements, accompanied by molecular dynamics simulations, the built-up of the particle shells, in general, and the PBI aggregation behavior, in particular, are explored in depth.

Synthesis and Electrochemical and Photophysical Properties of Azaterrylene Derivatives

A series of terrylene derivatives, such as monoazaterrylene (MATerry), 1,6-diazaterrylene (DiATerry) and pristine terrylene (Terry), were synthesized by changing the number of nitrogen atoms at the bay region (1 and 6 positions of the Terry core). The electrochemical measurements suggested that the first one-electron reduction and oxidation potentials became positively shifted with increasing numbers of nitrogen atoms. This agreed with the energies of the corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states estimated by DFT methods. In contrast, the HOMO-LUMO gaps approximately remained constant. This trend is quite similar to the spectroscopic behaviors observed by absorption and fluorescence spectra. The solvent polarity-dependent spectroscopic trends of DiATerry suggested the intramolecular charge-transfer (ICT) characters. The evaluation of the excited-state dynamics in various solvents indicated the electronic configurational changes of the excited states relative to the ground state via the ICT. This was supported by the Lippert-Mataga plots. Finally, the reversible protonation and deprotonation processes were also observed.

Multiexciton Dynamics Depending on Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs

Pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)₂ and m-(Pc)₂] were synthesized to examine intramolecular orientation-dependent multiexciton dynamics, especially focusing on singlet fission (SF) and recombination from correlated triplet pairs [(TT)]. Absorption and electrochemical measurements indicated strong intramolecular couplings of o-(Pc)₂ relative to m-(Pc)₂. Femtosecond and nanosecond TA measurements successfully demonstrated efficient SF in both dimers. In contrast, the dissociation process from the (TT) to the individual triplets [(2 × T)] was clearly observed in m-(Pc)₂, which is in sharp contrast to a major recombination process in o-(Pc)₂. Time-resolved electron spin resonance (TR-ESR) measurements demonstrated that the recombination and dissociation proceed from the quintet state of ⁵(TT) in m-(Pc)₂. The rate constant of the SF was 2 orders of magnitude greater in o-(Pc)₂ than that in m-(Pc)₂ and was rationalized by enhanced electronic coupling between adjacent HOMOs of the Pc units.