Templated assembly of photoswitches significantly increases the energy-storage capacity of solar thermal fuels Timothy J. Kucharski, Nicola Ferralis, Alexie M. Kolpak, Jennie O. Zheng, Daniel G. Nocera & Jeffrey C. Grossman Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Org. Lett., 2014, 16 (6), pp 1704–1707

Previous studies UV UV Vis or heat Cat. or heat R =1,1-timethyltridecyl norbornadiene/quadricyclane (fulvalene)diruthenium (FvRu2) system Kasper Moth-Poulsen, Dušan Ćoso, Karl Börjesson, Nikolai Vinokurov, Steven K. Meier, Arun Majumdar, K. Peter C. Vollhardt and Rachel A. Segalman Energy Environ. Sci., 2012,5, 8534-8537 Dubonosov, A. D., Bren, V. A. & Chernoivanov, V. A. Russ. Chem. Rev. 71, 917–927 (2002).

This approach Templating azobenzene-functionalized singlewalled carbon nanotubes (SWCNTs) with a functionalization density of one azobenzene chromophore for every eight SWCNT carbon atoms (noted as 1/8).

Synthesis Functionalization densities Azo- SWCNTs 1 1/37 Fig.1a, Synthetic scheme for the iterative functionalization of SWCNTs with azobenzenes.

Characterization Azo-4 Fig.1c, FTIR spectra of free azobenzene Azo-4, pristine SWCNTs and Azo-SWCNT 1. Fig.1d, Representative TGA plots of pristine SWCNTs and Azo-SWCNTs 1, 2 and 3 heated from 100 to 750C and held at 750C. Fig.1e, The difference in mass loss between samples of Azo-SWCNTs 1, 2 and 3 and pristine SWCNTs during the constant-rate decomposition of SWCNTs

Thermal isomerization kinetics Fig.2c, Time-resolved ultraviolet–visible spectra indicate the thermal cis-trans isomerization in the heated to 75C. Fig.2e, Time-resolved fluorescence spectra indicate thermal cis-trans isomerization in the sample with the stage heated to 80C. Fig.2a, Reaction scheme for photochemical charging and thermal activation of energy release in Azo-SWCNTs.

Cyclability Fig.3b, Optical cycling of Azo-SWCNT 3 in acetonitrile by alternatively irradiating at 350 and ≥450 nm. Fig.3c, Optical and thermal cycling of Azo-SWCNT 3 in acetonitrile by alternating periods of 350 nm irradiation and dark with the sample held at 75C.

Energy storage Fig.4a, Non-interacting Azo-SWCNTs in a suspension. Fig.4b, Interacting Azo-SWCNTs bundled in the solid state.

Energy storage Fig.5a, Enthalpy difference ΔHcis-trans for intercalated Azo-SWCNTs (blue diamonds), as well as total energy per azobenzene of the cis (cyan squares) and trans (red circles) conformations, as a function of SWCNT separation. Fig.5b, Computed atomic structures at the minimum-energy trans separation distance.

Conclusion A proof-of-principle system to achieve solar thermal fuels of high energy density by templating photoswichable molecules on rigid low- mass nanostructures. Steric strain in chromophore-template constructs can increase both the amount of energy stored per photochromic molecule by more than 200% and the strage lifetimes by orders of magnitude, along with resisting material degradation under repeated cycling.