Structure-Property Relationships in Photoresponsive Molecular Materials

Resumen

Small pi-conjugated organic materials are of great significance in materials science due to the versatility and superiority in practical applications, especially in opto-electronics. Among the numerous types of small organic materials, those essentially based on the structural motif of the cyano-vinyl synthon have obtained intensive and long-term attention, motivated by the striking behaviors such as the strong luminescent enhancement in the solid state, the prominent twist elasticity of the floppy structure, as well as the good photovoltaic performance. Although many CN-substituted distyrylbenzenes (DCS)-type materials have been successfully developed and applied, an in-depth understanding of the structural factors responsible for these particularities is rather limited by now, which has enormous implications for the desired approach of targeted design, to avoid expensive and tedious trial and error synthetic approaches. In this thesis, the pronounced solid state luminescence enhancement (SLE) and the twist elasticity (TE), as well as the high power conversion efficiency (PCE) of organic solar cells (OSCs) based on the cyano-stilbene motif platform, are investigated integrating experimental steady-state & transient absorption and fluorescence measurements and quantum-chemical calculations. As SLE-active materials with emerging applications in organic (opto)electronics, the issue of SLE, popularized under the (however somewhat misleading) term aggregation-induced emission (AIE), has evolved as a hot field in current materials research. The first part of the thesis will be dedicated to understand the SLE phenomenon derived from one-component systems based on a library of multiple CN-substituted distyrylbenzenes (DCS family) with systematically, subtly varying substitution pattern in terms of type and position. Depending on positional isomerism, these compounds exhibit dramatically changed photoresponse upon changes of the environment, going from fluid solution to solid solution and to the single crystalline phase. Moreover, pronounced solid state color shifts are observed even in one and the same compound due to polymorphism. To achieve a full understanding of all these observations, the current work first provides a detailed conceptual analysis of all contributing factors for structural polymorphism, for solid state shifts and SLE, which includes geometrical vs. electronic factors, intra- vs. intermolecular contributions, radiative vs. nonradiative decay channels. Then, by combining quantitative (ultra)fast optical spectroscopic techniques, appropriate quantum-chemical methods, and structural (X-ray) data, these contributions are systematically isolated, analyzed and quantified. In all, this allows for a full understanding of the twist-elasticity concept, of the solid state color shifts, but in particular provides a first holistic picture of SLE, where all details involved in the SLE process are fully elucidated and rationalized. The second part of this thesis will go one step further in materials, addressing specific device applications (i.e. OSCs), to investigate a two-component system made of small molecules and used in a bulk heterojunction (BHJ) OSC. This study is driven by the rapid development of solution-processed fullerene-free all-small-molecule OSCs (ASM-OSC), being a novel, highly promising alternative route to the classical polymer-based OSCs. Although improved stability and high PCE of this new type of cells was demonstrated, the reasons for the good performance of ASM-OSCs are however not clear yet; however, they must be known in order to fully exploit the design capacities of these materials. Here we use a combined computational-experimental approach to study the photoexcitation dynamics of a prototypical all-small-molecule photovoltaic blend, namely p-DTS(FBTTh2)2 as donor and NIDCS-MO as acceptor. Despite high PCE and high open circuit voltages, only very weak excitonic coupling and localized charge transfer (CT) states, and slow exciton dissociation rates are observed, due to even slower parasitic exciton quenching, enabled by the relatively high purity level. The analysis in the current work clearly proves that the morphological and chemical purity of the small molecules, which distinguishes them from classic polymers, slows down parasitic quenching and plays the key role for the good performance of ASM-OSCs. The device behaves like a quasi-ideal cell with negligible activation for charge generation but high activation for charge recombination, which permits to separate the localized CT states.