Photoreversible Transformation between Seconds and Hours Time-Scales: Threading of Pillar[5]arene onto the Azobenzene-End of a Viologen Derivative

Photoswitching of the transformation between seconds and hours time-scales is demonstrated using the threading of per-hydroxylated pillar[5]­arene onto the azobenzene-end of a viologen derivative. When the azobenzene moiety was in the trans form, the threading of per-hydroxylated pillar[5]­arene quickly took place at 25 °C and could not be monitored directly. The exchange rate (k) and half-life time (t1/2) examined by 2D EXSY NMR spectroscopy were found to be 0.209 ± 0.013 s–1 and 3.33 ± 0.21 s, respectively. In contrast, the cis form of the azobenzene moiety required very long time (k = 2.14 ± 0.27 × 10–5 s–1, t1/2 = 9.13 ± 1.2 h) to thread per-hydroxylated pillar[5]­arene at 25 °C. Photoisomerization from the trans to the cis form generated the following increment of free energy of activation at 25 °C: ΔG in ‡ (cis form) – ΔG in ‡ (trans form) = 22.8 ± 0.24 kJ mol–1, which led to the time-scale transformation. The tuning of the threading was also accomplished by heating/cooling: the rate constants increased on heating and decreased on cooling.