Storing solar energy is a vital component of using renewable energy sources to meet the growing demands of the global energy economy. Molecular solar thermal (MOST) energy storage is a promising means to store solar energy with on-demand energy release. The light-induced isomerization reaction of norbornadiene (NBD) to quadricyclane (QC) is of great interest because of the generally high energy storage density (0.97 MJᐧkg–1) and long thermal reversion lifetime (t1/2, 300K = 8346 years). However, the mechanistic details of the ultrafast excited-state [2+2]-cycloaddition is largely unknown due to the limitations of experimental techniques in resolving accurate excited-state molecular structures. We now present a full computational study on the excited-state deactivation mechanism of NBD in the gas phase. Our multiconfigurational calculations [SA6-CASSCF(4,7)/ANO-S-VDZP] and non-adiabatic molecular dynamics simulations have enumerated the possible pathways with 600 S2 initial conditions for 300 fs. The predicted S2 and S1 lifetimes are reported (62 and 221 fs). The QC: NBD formation ratio is 1:5; the predicted quantum yield of QC is 9%, which underscores the potential of NBD for MOST materials. Our simulations also show the mechanisms of forming other possible reaction products and their quantum yields.
Nanothreads are an emerging one-dimensional sp3-hybridized material with high predicted tensile strength and a tunable band gap. They can be synthesized by compressing aromatic, or non-aromatic small molecules under 15-30 GPa of pressure. Recently, new avenues are being sought that reduce the pressure required to afford nanothreads; focus has been placed on the polymerization of molecules with reduced aromaticity, favorable stacking, and/or the use of higher reaction temperatures. Herein, we report the photochemically-mediated polymerization of pyridine and furan aromatic precursors, which achieves nanothread formation at reduced pressures. In the case of pyridine, it was found that a combination of slow compression/decompression with broadband UV light exposure yielded a crystalline product featuring a six-fold diffraction pattern with similar interplanar spacings of previously synthesized pyridine-derived nanothreads at a reduced pressure. When furan is compressed to 8 GPa and exposed to broadband UV light, a crystalline solid is recovered that similarly demonstrates X-ray diffraction with an interplanar spacing akin to that of the high-pressure synthesized furan-derived nanothreads. Our method realizes a 1.9-fold reduction in the maximum pressure required to afford furan-derived nanothreads and a 1.4-fold reduction in pressure required pyridine-derived nanothreads. Density functional theory and multiconfigurational wavefunction-based computations were used to understand the photochemical activation of furan and subsequent cascade thermal cycloadditions. The reduction of the onset pressure is caused by an initial [4+4]-cycloaddition followed by increasingly facile thermal [4+2]-cycloadditions during polymerization. Density functional theory and multiconfigurational wavefunction-based computations were used to understand the photochemical activation of furan and subsequent cascade thermal cycloadditions.
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