Chemical vapor deposition allows the preparation of few-layer films of MoTe in three distinct structural phases depending on the growth quench temperature: 2H, 1T', and 1T. We present experimental and computed Raman spectra for each of the phases and utilize transport measurements to explore the properties of the 1T MoTe phase. Density functional theory modeling predicts a (semi-)metallic character. Our experimental 1T films affirm the former, show facile μA-scale source-drain currents, and increase in conductivity with temperature, different from the 1T' phase. Variation of the growth method allows the formation of hybrid films of mixed phases that exhibit susceptibility to gating and significantly increased conductivity.

In the photoluminescence spectra of thin films made of singlet fission (SF) materials emission features that are red-shifted from the free exciton emission are of particular interest. They can be fingerprints of the correlated triplet-pair state and as such offer insights into the mechanisms of the multistep SF process. However, excimer formation or trap-state population can also cause such features and a clear disentanglement of the various contributions can be challenging. Here, we use blends of anthradithiophene (ADT) and weakly interacting organic semiconductors to control the polarizability of the molecular environment and, thus, to distinguish between excimer emission and emission from the correlated triplet-pair state. Using time-resolved photoluminescence spectroscopy measurements, we clarify the relation between excimer formation and SF in ADT and find that excimer formation constitutes a parallel relaxation channel for the exciton and neither mediates nor hinders SF.

Exposure of a solution of the square pyramidal tungstacyclopentane complex W­(NAr)­(OSiPh3)2(C4H8) (Ar = 2,6-i-Pr2C6H3) to ethylene at 22 °C in ambient (fluorescent) light slowly leads to the formation of propylene and the square pyramidal tungstacyclobutane complex W­(NAr)­(OSiPh3)2(C3H6). No reaction takes place in the dark, but the reaction is >90% complete in ∼15 min under blue LED light (∼450 nm λmax). The intermediates are proposed to be (first) an α methyl tungstacyclobutane complex (W­(NAr)­(OSiPh3)2(αMeC3H5)), and then from it, a β methyl version. The TBP versions of each can lose propylene and form a methylene complex, and in the presence of ethylene, the unsubstituted tungstacyclobutane complex W­(NAr)­(OSiPh3)2(C3H6). The W–Cα bond in an unobservable TBP W­(NAr)­(OSiPh3)2(C4H8) isomer in which the C4H8 ring is equatorial is proposed to be cleaved homolytically by light. A hydrogen atom moves or is moved from C3 to the terminal C4 carbon in the butyl chain as the bond between W and C3 forms to give the TBP α methyl tungstacyclobutane complex. Essentially, the same behavior is observed for W­(NCPh3)­(OSiPh3)2(C4H8) as for W­(NAr)­(OSiPh3)2(C4H8), except that the rate of consumption of W­(NCPh3)­(OSiPh3)2(C4H8) is about half that of W­(NAr)­(OSiPh3)2(C4H8). In this case, an α methyl-substituted tungstacyclobutane intermediate is observed, and the overall rate of formation of W­(NCPh3)­(OSiPh3)2(C3H6) and propylene from W­(NCPh3)­(OSiPh3)2(C4H8) is ∼20 times slower than in the NAr system. These results constitute the first experimentally documented examples of forming a metallacyclobutane ring from a metallacyclopentane ring (ring contraction) and establish how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene. They also raise the possibility that ambient light could play a role in some metathesis reactions that involve ethylene and tungsten-based imido alkylidene olefin metathesis catalysts, if not others.

Covalent assemblies of conjugated organic chromophores provide the opportunity to engineer new excited states with novel properties. In this work, a newly developed triple-stranded cage architecture, in which meta-substituted aromatic caps serve as covalent linking groups that attach to both top and bottom of the conjugated molecule walls, is used to tune the properties of thiophene oligomer assemblies. Benzene-capped and triazine-capped 5,5′-(2,2-bithiophene)-containing arylene cages are synthesized and characterized using steady-state and time-resolved spectroscopic methods. The conformational freedom and electronic states are analyzed using time-dependent density functional theory. The benzene cap acts as a passive spacer whose electronic states do not mix with those of the chromophore walls. The excited state properties are dominated by through-space interactions between the chromophore subunits, generating a neutral Frenkel H-type exciton state. This excitonic state undergoes intersystem crossing on a 200 ps time scale while the fluorescence output is suppressed by a factor of 2 due to a decreased radiative rate. Switching to a triazine cap enables electron transfer from the chromophore-linker after the initial excitation to the exciton state, leading to the formation of a charge-transfer state within 10 ps. This state can avoid intersystem crossing and exhibits red-shifted fluorescence with enhanced quantum yield. The ability to interchange structural modules with different electronic properties while retaining the overall cage morphology provides a new approach for tuning the properties of discrete chromophore assemblies.

The technique of focused ion beam (FIB) milling is used to cut twodimensional shapes into single crystals composed of the organic semiconductor molecule perylene. The use of an ultrathin Au coating and its removal using a KI/I 2 etchant solution allow the crystal to be imaged in the FIB apparatus while protecting it from electron beam damage. Using this approach, features with a spatial resolution on the order of 130 nm can be created while retaining 90% of the original photoluminescence intensity in the surrounding crystal regions. These proof-of-principle experiments demonstrate that FIB milling could provide a general way to control the nanoscale morphology of organic molecular crystals.