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

Abstract

A series of β-TCBD (1,1,4,4-tetracyano-buta-1,3diene)-appended porphyrins, M-TCBD (M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)), was synthesized from 2,3-diphenylethynyl-12nitro-meso-tetraphenylporphyrin, H 2 -PE 2 , 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 -PE 2 . 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 β-position of the porphyrin macrocycle. Notably, these porphyrins exhibited three porphyrin ring-centered reductions due to extended π-conjugation. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H 2 -PE 2 ) 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. Zscan measurements revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrins. The two-photon absorption coefficient (β) and the imaginary part of third-order nonlinear optical susceptibility (χ (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 χ (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 × 10 15 W/m 2 ) of the synthesized porphyrins makes them good materials for laser protection and high-power laser operation.

Unsymmetrically β-Functionalized π-Extended Porphyrins: Synthesis, Spectral, Electrochemical Redox Properties, and Their Utilization as Efficient Two-Photon Absorbers

“…Unlike many other porphyrins, these asymmetric disubstituted porphyrins have dominant B-band absorption features. 75,76 The individual optical absorption spectra of all synthesized porphyrins are shown in Figures S1−S10 in the Supporting Information (SI). To elucidate the effect of peripheral substitution and the role of nonplanarity in the emission profile of the synthesized porphyrins, the free-base and Zn(II) complexes were characterized by fluorescence spectroscopy.…”

“…They documented the nonlinear optical parameters of various similar types of porphyrins as a comparison, and our current study exhibited superior responses over all of the referred macrocycles as a nonlinear optical material. Recently, Rohal et al 76 have explored nonlinear optical behavior of β-trisubstituted porphyrins with three different peripheral residues and claimed that their materials have the highest optical limiting threshold (∼10 15 W/m 2 ) to date in the porphyrin chemistry under laser parameter 800 nm central wavelength operating at a 1 kHz repetition rate of pulse duration 120 fs. They argued that the strong electron-withdrawing β-substituents like 1,1,4,4-tetracyano-buta-1,3-diene (TCBD) and the nitro group make the porphyrin a better chromophore for optical sensors and protecting human eyes from ultrafast and high-intensity lasers.…”

“…On the other hand, ∼7 and ∼4 nm blueshifts were observed for 1Ni and 2Ni in their last Q-bands, respectively, possibly due to the stabilization of the highest occupied molecular orbital (HOMO). Unlike many other porphyrins, these asymmetric disubstituted porphyrins have dominant B-band absorption features. , The individual optical absorption spectra of all synthesized porphyrins are shown in Figures S1–S10 in the Supporting Information (SI).…”

“…75 Lately, Rohal and co-workers documented electron-deficient unsymmetrical β-TCBD-appended trisubstituted porphyrins with the highest optical limiting threshold and claimed that these porphyrins act as better chromophores for optical sensors and protectors from high-intense lasers. 76 The investigations of π-extended porphyrin have been mainly restricted to symmetrical structures. Reports of unsymmetrically substituted π-extended porphyrins are scarce due to their limited access.…”

“…Later on, Kumar and co-workers explored meso -substituted trans -A 2 BC-type porphyrins having phenothiazine moiety and o -/ p -nitrophenyl group showing “push–pull” effect and utilized them for two-photon absorption and ultrafast dynamics . Lately, Rohal and co-workers documented electron-deficient unsymmetrical β -TCBD-appended trisubstituted porphyrins with the highest optical limiting threshold and claimed that these porphyrins act as better chromophores for optical sensors and protectors from high-intense lasers …”