Final Report Summary - OPTELOMAC (Opto-Electronic Organic Materials by New Acetylene Chemistry)
The chemical structural landscape for organic molecules with desirable properties for application in opto-electronic devices has been rather limited so far: organic-based devices largely use components made of well-established building blocks. The discovery of new scaffolds and synthetic methodologies, on the other hand, promises new breakthroughs in the field of functional organic materials. We embarked on a program targeting new push-pull chromophores with the following properties: (i) intense, tunable, and bathochromically shifted charge-transfer absorptions covering the entire solar spectrum from the ultraviolet to the near infrared, (ii) high second-order hyperpolarizabilities and third-order optical nonlinearities, (iii) high thermal stability and sublimability for high-optical-quality film formation by vapor-phase deposition, and iv) versatile synthesis. With these compounds, device fabrication for optical data transmission and next-generation organic solar cells was pursued in interdisciplinary collaboration with physicists and material scientists. For chemists, to stay in the driver's seat in these interdisciplinary collaborations, it is important to also design and realize the properties such as high stability and sublimability, that are required up to the device stage.
To these ends, our research group developed a versatile synthesis of non-planar, stable, and often fully sublimable push-pull chromophores based on the [2+2] cycloaddition (CA) of a diversity of electron-rich alkynes and electron-poor olefins, followed by a retroelectrocyclization (RE). This click-chemistry-type, atom-economic, and high-yielding transformation has been applied to the preparation of a library of near 300 chromophores featuring intramolecular charge-transfer, redox-amphotheric behavior, and matching the materials property requirements outlined above. We expanded particularly the chemical space of the electron-accepting olefin, which initially consisted only of tetracyanoethene (TCNE) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ): the transformation works indeed well with dicyanovinyl and tricyanovinyl derivatives, N,N-dicyanoquinone diimides (DCQNIs), or 6,6-dicyanofulvenes. With electron-rich buta-1,3-diynes, double TCNE additions were achieved leading to unprecedented octacyano[4]dendralenes which exceed by far TCNQ in their propensity for reversible electron uptake. We investigated in a comprehensive and systematic way the physical properties of the neutral resulting chromophores as well as of their anion radicals. First applications in the construction of silicon-organic hybrid devices for extremely fast all-optical switching have been achieved in collaboration. Remarkably, the central double bond in [3]cumulenes shows proacetylenic reactivity and this allowed the preparation of dicyanodiaryltetracenes by initial cycloaddition of TCNE, in an unprecedented high-yielding 2-step synthesis. These strongly fluorescent compounds show outstanding singlet exciton fission properties that are of great interest for next-generation organic solar cell applications.
The second major research accomplishment was the demonstration that optically active 1,3-diethynylated allenes (DEAs) may have a fascinating future as shape-persistent chiral chromophores, whose chiral properties are as good, if not better, than those of the benchmark axially chiral scaffolds, such as binaphthyls or spirobifluorenes. We prepared two series of optically active acyclic and macrocyclic alleno-acetylene oligomers with all-carbon backbones, displaying some of the most intense Cotton effects in their electronic circular dichroism spectra (ECD) known today, and analyzed the origin of these chiroptical properties in great detail. General rules were established in systematic study of alleno-acetylenic macrocycles for obtaining intense ECD responses in carbon-rich matter: (i) use chromophores with high extinction coefficient and high transition dipole moments, (ii) restrict their conformations by rendering them shape-persistent, and (iii) design the systems so they belong to a highly symmetrical chiral point group (Dn). We introduced DEAs as novel chiral building blocks into supramolecular systems and prepared microporous materials featuring chiral channels, which also assemble in aqueous solution. Furthermore, we prepared enantiopure alleno-acetylenic helicates ("helicages") which enable the chiroptical detection of non-chromophoric achiral guests, such as 1,4-dioxane, bound to their interior cavity.