Random rotational fluctuation of solvents generates a large transient electric field that breaks the symmetry required for symmetry breaking charge transfer.

Photoactive metal–organic framework (MOF) thin films offer an opportunity for translating the advantages of periodic, crystalline, and tailorable light-harvesting materials directly into devices such as those for photoelectrochemical solar energy conversion. In this study, we report the fabrication of light-absorbing perylene-diimide-containing pillared-paddlewheel MOF thin films using an automated layer-by-layer (LbL) deposition technique. Our focus here is on optimizing the growth of representative chromophoric MOFs as oriented films of uniform and predefined thickness. Growth was examined as a function of metal identity, pillaring ligand composition, and supporting-surface chemical functionality. Application of atomic force microscopy (AFM) and complementary techniques revealed that the surface-supported MOFs initially display island-type film growth (Volmer–Weber growth), resulting in comparatively rough films. Further growth is accompanied by the merging of islands, resulting in films that, depending on experimental details, can be remarkably smooth (i.e., roughness on the order of ±1 nm (one structural repeat unit in the pillaring direction)). These details include the use of 1,4-diazabicyclo[2.2.2]octane (DABCO) as a MOF pillar and ALD-grown zinc oxide as a film support (ALD = atomic layer deposition). Also helpful for mitigating island-type growth, at least in part, is the replacement of Zn2+ by Cu2+ as the metal component of the MOF. Notably, each of these adjustments entails replacing weaker chemical bonds with stronger ones.

Mixed-dimensional van der Waals heterojunctions involve interfacing materials with different dimensionalities, such as a 2D transition metal dichalcogenide (TMD) and a 0D organic semiconductor. These heterojunctions have shown unique interfacial properties not found in either individual components. Here, we use femtosecond transient absorption to reveal photoinduced charge transfer and interlayer exciton formation in a mixed-dimensional heterojunction between monolayer MoS2 and vanadyl phthalocyanine (VOPc). We find that hole transfer occurs from the MoS2 valence band to VOPc in ~710 fs, while electron transfer occurs from VOPc excited state to MoS2 conduction band in less than 100 fs. This ultrafast electron or hole transfer results in the formation of an interlayer exciton across the interface. Additionally, we observe the emergence of a new absorption band that is formed only in the heterojunction and assign it to the formation of delocalized hybrid states between the VOPc and MoS2 layers. By directly exciting this band, an interlayer exciton forms that is not limited by electron or hole transfer times. These interlayer excitons across the interface lead to longer-lived charge-separated states of up to 2.5 ns, longer than in each individual layer of this heterojunction. Thus, the longer charge-separated state along with ultrafast charge transfer times provide promising results for photovoltaic and optoelectronic device applications.

A trimer of dicyanoaurate has been studied as a model system of the covalent chemical bond formation. Here, we report the dynamics of dicyanoaurate trimer in water upon photoexcitation by femtosecond time-resolved luminescence (TL) and luminescence spectra at cyrogenic temperature. Temperature was varied as a means to control the medium flexibility as well as the population of isomers. A unique parallelism between the luminescence spectrum vs. time and vs. temperature was observed, which enables unambiguous luminescence band assignments and facilitates investigation of the dynamics. Upon photoexcitation to S 1 , intersystem crossing proceeds in an ultrafast manner within 20 fs due to the large spin−orbit coupling followed by a structural change from a loose bent to a tight linear form in 1.5 ps. Higher oligomerization occurs above the melting temperature. TL reveals a strong coherent excitation of the symmetric Au−Au stretching vibration at 74 cm −1 through the non-Condon effect.

Internal conversion is the first step after photoexcitation to high lying electronic states, and plays a central role in many photoinduced processes. In this report, we demonstrate truly ultrafast internal...