β-Tetracyanobutadiene-Appended Porphyrins: Facile Synthesis, Spectral and Electrochemical Redox Properties, and Their Utilization as Excellent Optical Limiters

A series of beta-TCBD (1,1,4,4-tetracyano-buta-1,3-diene)-appended porphyrins, M-TCBD (M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)), was synthesized from 2,3-diphenylethynyl-12-nitro-meso-tetraphenylporphyrin, H-2-PE2, and characterized by various spectroscopic techniques and electrochemical studies. The reaction proceeds via [2 + 2] cycloaddition and retroelectrocyclization reactions of tetracyanoethylene (TCNE) with H-2-PE2. The observed unusual reduction potentials in the cyclic voltammograms of the synthesized porphyrins in the range of -0.06 to -0.10 V are the consequence of the TCBD moiety present at the beta-position of the porphyrin macrocycle. Notably, these porphyrins exhibited three porphyrin ring-centered reductions due to extended pi-conjugation. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H-2-PE2) was attributed to the existence of intramolecular charge transfer and enhanced polarization in the M-TCBD series. The single-beam femtosecond Z-scan measurements were performed to elucidate the third-order nonlinear optical properties, and the temporal response of these porphyrin molecules was investigated using optical pump-probe spectroscopy to study the excited state absorption dynamics. Z-scan measurements revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrins. The two-photon absorption coefficient (beta) and the imaginary part of third-order nonlinear optical susceptibility (chi((3))) were obtained from the open aperture experiment, whereas the close aperture experiment delivered the magnitude and the sign of the nonlinear refractive index (n(2)) and the real part of chi((3)). Furthermore, the femtosecond transient absorption spectroscopy revealed a faster relaxation dynamics of various absorption processes in a picosecond timescale. The excellent optical limiting threshold (1.90-2.33 x 10(15) W/m(2)) of the synthesized porphyrins makes them good materials for laser protection and high-power laser operation.

Automated summary

A series of beta-TCBD-appended porphyrins were synthesized and characterized for their nonlinear optical response and excited state absorption dynamics, showing potential applications in laser protection and high-power laser operation. The obtained results indicated enhanced nonlinear optical properties and faster relaxation dynamics, making the synthesized porphyrins promising materials for future optical applications.