Aza-bicyclooctadiene/tetracyclooctane couples as promising photoswitches for molecular solar thermal energy storage applications†

The scrutiny of molecular photoswitches has received utmost attention owing to their plethora of promising applications. A bicyclooctadiene/tetracyclooctane (BOD/TCO) couple has recently been recognized as a suitable photoswitching system for molecular solar thermal energy storage (MOST). However, there is a desirable interest in tuning the properties of the BOD/TCO couple for enhanced performance. In the present report, a systematic attempt has been made to unravel the photoswitching properties of various aza-BOD/TCO systems using density functional theory (DFT) studies. DLPNO-CCSD(T)/def2TZVP calculations were performed to test the reliability of the obtained outcomes. Attention has also been devoted to assess the effect of substitution and solvation on the photoswitching behaviour. The result reveals that the aza-BOD/TCO couples are versatile systems whose properties substantially depend on the position of N. The substitution of N at the bridgehead position (1-aza-BOD/TCO and 1,4-biaza-BOD/TCO) enhances the barrier height for the thermal back isomerization reaction, whereas substituting the unsaturated C with N (2-aza-BOD/TCO) and the bridgehead and closest unsaturated C with N (1,2-biaza-BOD/TCO) improves the storage density and photophysical properties. The 2-aza-BOD/TCO couple has a storage energy of 213.05 kJ mol^?1 (1.99 MJ kg^?1) which is significantly higher than the parent BOD/TCO couple (163.85 kJ mol^?1 (1.54 MJ kg^?1)). Biaza-TCO has the highest barrier of 200.00 kJ mol^?1 for the thermal back conversion reaction. The spectral overlap is reduced, and an 82.79 nm separation is achieved between the first important excitation wavelengths of substituted aza-BOD and aza-TCO. An approximately 32–109 nm red-shift in the first important excitation wavelength is noticed upon substitution of various aza-BODs and aza-TCOs.