Molecular solar-thermal energy storage systems are based on molecular switches that reversibly convert solar energy into chemical energy. Herein, we report the synthesis, characterization, and computational evaluation of a series of low molecular weight (193-260 g mol(-1)) norbornadiene-quadricyclane systems. The molecules feature cyano acceptor and ethynyl-substituted aromatic donor groups, leading to a good match with solar irradiation, quantitative photo-thermal conversion between the norbornadiene and quadricyclane, as well as high energy storage densities (396-629 kJ kg(-1)). The spectroscopic properties and energy storage capability have been further evaluated through density functional theory calculations, which indicate that the ethynyl moiety plays a critical role in obtaining the high oscillator strengths seen for these molecules.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201602530

Low Molecular Weight Norbornadiene Derivatives for Molecular Solar-Thermal Energy Storage.

Developing norbornadiene–quadricyclane (NBD–QC) systems for molecular solar-thermal (MOST) energy storage is often a process of trial and error. By studying a series of norbornadienes (NBD-R2) doubly substituted at the C7-position with R = H, Me, and iPr, we untangle the interrelated factors affecting MOST performance through a combination of experiment and theory. Increasing the steric bulk along the NBD-R2 series gave higher quantum yields, slightly red-shifted absorptions, and longer thermal lifetimes of the energy-rich QC isomer. However, these advantages are counterbalanced by lower energy storage capacities, and overall R = Me appears most promising for short-term MOST applications. Computationally we find that it is the destabilization of the NBD isomer over the QC isomer with increasing steric bulk that is responsible for most of the observed trends and we can also predict the relative quantum yields by characterizing the S1/S0 conical intersections. The significantly increased thermal half-life of NBD-iPr2 is caused by a higher activation entropy, highlighting a novel strategy to improve thermal half-lives of MOST compounds and other photo-switchable molecules without affecting their electronic properties. The potential of the NBD-R2 compounds in devices is also explored, demonstrating a solar energy storage efficiency of up to 0.2%. Finally, we show how the insights gained in this study can be used to identify strategies to improve already existing NBD–QC systems.

Unraveling factors leading to efficient norbornadiene–quadricyclane molecular solar-thermal energy storage systems

In this article we discuss synthetic routes to organic conjugated oligomers and polymers bearing triple C-C bonds that have been recently developed in our laboratories, based on Pd-catalyzed Csp-Csp 2 coupling reactions. Experimental protocols have been tuned to face synthetic challenges such as the presence in the main conjugated backbone of multifunctional substituents or uncommon conju- gated units. The relevance of the obtained products in terms of properties and applications is also shortly mentioned.

Synthesis of Functionalized Aryleneethynylene Oligomers and Polymers for Organic Electronics by Pd-Catalyzed Coupling Reactions

Single-molecule fluorescence emission of certain positive photochromic systems such as diarylethenes have been exploited for biological imaging and optical memory storage applications. However, there is a lack of understanding if negative photochromic systems can be used for such type of applications. Hence, to explore the potential of negative photochromic molecules for possible optical memory storage applications, we have here synthesized and studied a series of four norbornadiene–quadricyclane (NBD–QC) photoswitching molecules. These molecules feature either linearly conjugated or cross-conjugated pi-electron systems. Upon photoisomerization, the UV-vis absorption spectra of the molecules revealed a strong blue shift in the QC-form, with a photoisomerization quantum yield close to 80% for the cross-conjugated systems. In contrast, a strong intrinsic emission (up to Φf = 49%) for the linearly conjugated compounds in the NBD form was observed. Upon light-induced isomerization, the emission was completely turned off in the QC-form in all the compounds studied. Further, the robustness of the system was evaluated by performing several switching cycles. Under nitrogen, the emission can be turned off and recovered with almost no loss of emission. We also show that the QC-form can be photochemically triggered to convert back to the NBD-form using a low energy UV light (340 nm), allowing an all optical conversion to both species. The demonstrated properties can make the NBD–QC system attractive for potential applications such as optical memory storage devices.

/pdf/turn-off-mode-fluorescent-norbornadiene-based-photoswitches-1tyqusrem5.pdf

Turn-off mode fluorescent norbornadiene-based photoswitches

This chapter presents the norbornadiene (NBD) molecule as a photoswitch. This bicyclic molecule is photoconverted to a high-energy metastable isomer quadricyclane (QC) via light-induced [2+2] cycloaddition reaction. NBD is widely used as a molecular photoswitch due to its easily tunable isomerization properties (e.g. isomerization quantum yield, onset of absorption, half-life of the quadricyclane, and energy-storage density) by the functionalization with different chromophoric substituents. The molecular solar thermal-energy-storage system (MOST) application and its design of heat-release devices based on the NBD/QC system have been strongly evolving in the last years. However, also several new applications start to develop, making this photoswitch a versatile compound. 

Norbornadiene/Quadricyclane ( <scp>NBD</scp> / <scp>QC</scp> ) and Conversion of Solar Energy

A conjugated alternating copolymer containing norbornadiene and bis(ethynylene)phenylene units was prepared by the Cassar-Heck-Sonogashira cross-coupling reaction. Its electroluminescence was tested in a device, and its fluorescence colour could be tuned by light-induced norbornadiene-quadricyclane isomerization.

/pdf/photochemical-tuning-of-light-emission-in-a-conjugated-1k2io4aztd.pdf

Gianmarco Bilancia

Papers

Photochemical tuning of light emission in a conjugated polymer containing norbornadiene units in the main chain