Optical materials: Studying the role of hetropolyacid to enhance the nonlinear optical responses of porphyrin in their hybrids system

Abstract

Nonlinear optical responses are measured to investigate the role of hetropolyacid (HPA) in porphyrin-Lindqvist type {∼PM@HPA} hybrid systems, {where PM@HPA = [(TBA)8{(MTPPN4)(Mo6O18)4}]; M = 2H (1) and Mn (2)}. The well-known Z-scan technique with wavelength (λ) = 532 nm and pulse duration of τ = 6 ns is used. The results clearly show the significant enhancement in nonlinear self-defocusing effect and reverse saturable absorption under closed and open-aperture configuration for PM@HPA hybrids. It is also observed that the second hyperpolarizability γ values, {1 (4.639 × 10−29 esu) < 2 (5.214 × 10−29 esu)}, are directly proportional to the discrepancy of LUMO level (ΔE) values {1 (2.214 eV) < 2 (2.339 eV)} while they are inversely proportional to the HOMO−LUMO energy gap Eg values {1 (2.003 eV) > 2 (1.862 eV)} in the PM@HPA hybrid system where the porphyrin moiety acts as electron donor whilst HPA acts as electron acceptor. Therefore, the emergence of a facile electron transformation from the porphyrin moieties to the low-lying LUMO levels of HPA is mainly responsible for the enhancement in nonlinear optical responses of PM@HPA hybrids system. As a consequence the hybrid compounds can be fascinating for optical limiting applications.