Encapsulation of TCNQ and the Acridinium Ion within a Bisporphyrin Cavity: Synthesis, Structure, and Photophysical and HOMO-LUMO-Gap-Mediated Electron-Transfer Properties

The encapsulation of tetracyanoquinodimethane (TCNQ) and fluorescent probe acridinium ions (AcH+) by diethylpyrrole-bridged bisporphyrin (H4DEP) was used to investigate the structural and spectroscopic changes within the bisporphyrin cavity upon substrate binding. X-ray diffraction studies of the bisporphyrin host (H4DEP) and the encapsulated hostguest complexes (H4DEP.TCNQ and [H4DEP.AcH]ClO4) are reported. Negative and positive shifts of the reduction and oxidation potentials, respectively, indicated that it was difficult to reduce/oxidize the encapsulated complexes. The emission intensities of bisporphyrin, upon excitation at 560 nm, were quenched by about 65?% and 95?% in H4DEP.TCNQ and [H4DEP.AcH]ClO4, respectively, owing to photoinduced electron transfer from the excited state of the bisporphyrin to TCNQ/AcH+; this result was also supported by DFT calculations. Moreover, the fluorescence intensity of encapsulated AcH+ (excited at 340 nm) was also remarkably quenched compared to the free ions, owing to photoinduced singlet-to-singlet energy transfer from AcH+ to bisporphyrin. Thus, AcH+ acted as both an acceptor and a donor, depending on which part of the chromophore was excited in the hostguest complex. The electrochemically evaluated HOMOLUMO gap was 0.71 and 1.42 eV in H4DEP.TCNQ and [H4DEP.AcH]ClO4, respectively, whilst the gap was 2.12 eV in H4DEP. The extremely low HOMOLUMO gap in H4DEP.TCNQ led to facile electron transfer from the host to the guest, which was manifested in the lowering of the CN stretching frequency (in the solid state) in the IR spectra, a strong radical signal in the EPR spectra at 77 K, and also the presence of low-energy bands in the UV/Vis spectra (in the solution phase). Such an efficient transfer was only possible when the donor and acceptor moieties were in close proximity to one another.