Synthesis, spectroscopic, electrochemical redox, solvatochromism and anion binding properties of β-tetra- and -octaphenylethynyl substituted meso-tetraphenylporphyrins

Abstract

β-Tetra/octa-phenylethynyl substituted porphyrins and their metal (Co(II), Ni(II), Cu(II) and Zn(II)) complexes have been synthesized and characterized. H₂TPP(PE)₈ exhibited a remarkable red shift in the Soret (Δλ_max = 92 nm) band and longest wavelength band, Q_x(0,0) (Δλ_max = 117 nm), as compared to H₂TPP. Interestingly, MTPP(PE)₈ exhibited a 450–500 mV anodic shift in the first ring reduction potentials as compared to MTPP which is ascribed to the electron accepting nature of phenylethynyl groups and extended π-conjugation whereas the first ring oxidation potentials remain unaltered. Hence, we observed a significant reduction in the HOMO–LUMO gap. The fluorescence quantum yields and lifetime values of the free base and Zn(II) porphyrins are found to be 10–20 and 2–6 times, respectively lower than MTPPs. These porphyrins display a strong solvatochromism that is reflected by a large red-shift in their absorption and emission maxima upon increasing the solvent polarity. These porphyrins exhibited lower radiative rate constants (k_r) and enhanced nonradiative rate constants (k_nr) as compared to MTPPs. The decrement in fluorescence lifetime values, quantum yields, radiative rate constants (k_r) and profound solvatochromism with enhanced nonradiative rate constants (k_nr) have been interpreted in terms of intramolecular charge transfer (ICT) from the porphyrin core to phenylethynyl moieties and extended π-conjugation. Further, ZnTPP(PE)₈ was utilized for the colorimetric “naked-eye” sensing of CN⁻, F⁻, Cl⁻, Br⁻, H₂PO₄⁻ and CH₃COO⁻ ions through axial coordination of anions to Zn(II) metal centre. The electron deficient nature of the Zn(II) centre of ZnTPP(PE)₈ was further proved by axial ligation studies with anions in toluene. ZnTPP(PE)₈ exhibited extremely high binding constants (10¹² to 10⁷ M⁻²) with anions.