Fig. 4. Photocontrolled capture and release of DTF (5) by molecular tweezers 4 and light control of charge transfer from the host to the guest. (a) Schematic representation of visible light induced capture and release of guest molecule 5 by tweezers 4. Light of 405 nm wavelength induces release of 5. 470 nm light induces capture of 5. (b) Aromatic section of 1H-NMR spectra (400 MHz, CDCl3, 20 °C) recorded to assess binding between 5 and 4. Spectra of pure guest 5 and a 1 : 1 Z : E isomeric mixture of pure tweezers 4 are shown for comparison. No proton signal shifts are observed when combining pure E-4 and 5. Photoswitching of E-4 with 470 nm light induces strong shifts of the proton signals of 5. Photoswitching of 4 with 365 nm light reverses the shift of the proton signals of 5 and restores them to their original position. (c) Photographs illustrating the color changes associated CT modulations upon catch and release of 5 by tweezers 4 in CHCl3 solution. Addition of 5 to a solution of pure Z-4 induces a strong color shift to red, originating from the CT band of the formed Z-4·5 complex. Addition of 5 to E-4 does not induce a color change. Photoisomerization of the respective solutions cause color changes to lighter and deeper orange. (d) Van't Hoff plot depicting the temperature dependence of the binding between Z-4 and 5 enabling determination of ΔH and ΔS contributions. (e) Comparison of the measured binding constants Ka for association of different guest molecules 5–8 with tweezers Z-3 (grey bars) and Z-4 (orange bars). Ka values for guests 5 and 7 were obtained in CDCl3 and for guests 6 and 8 in CD2Cl2 solutions. Ka values for guest 5 are compared at 253 K and for all other guests at 293 K.
