Nano-second life time decay and Z-scan measurement: A tool to investigate the relevance of excited state dynamics with non-linear optical parameters in cis substituted Wells-Dawson POM-Porphyrin hybrids

Abstract

Materials possessing non-linear optical NLO responses have gained considerable attention in the field of photonics because they have the potential to produce new frequencies when they interact with intense light by means of non-linear interactions. Synthesizing NLO materials based on porphyrin covalently bonded with polyoxometalates (POMs) is complicated and rigorous, requiring special techniques in order to get pure compounds with a higher yield. Here, we study the third-order NLO parameters along with the fluorescence lifetime decay values in a nanosecond(ns) time span for WDPOMCis2PyP and WDPOMCis2PhP (bearing the Wells-Dawson POM at the cis position) and their porphyrin precursors Cis2PyPTris and Cis2PhPTris, respectively. A comparison of third-order NLO characteristics with fluorescence lifetime decay data indicates that an increase in NLO properties has been observed with shorter lifetime decay values and vice versa. From the results, it is clear that WDPOMCis2PhP showed better third-order NLO (χ3 = 2.60 × 10−10 esu and β = 8.26 × 10−11 cm/W) than WDPOMCis2PyP (χ3 = 2.25 × 10−10 esu and β = 5.60 × 10−11 cm/W), which is supported by lifetime decay values. The lifetime decay value for WDPOMCis2PhP (τ1 = 2.77 ns) is shorter than WDPOMCis2PyP (τ1 = 6.02 ns). In addition to the factors mentioned based on substituent impact and molecular geometry, the WDPOMCis2PyP has a larger dipole moment than other POM-based porphyrin hybrids, which accounts for the improved third-order NLO response. Minor energy variance among singlet, triplet excited states, and charge-separated states favors quicker intersystem crossing of the WDPOMCis2PyP and WDPOMCis2PhP, which boosts NLO responses, and this actually makes WDPOMCis2PyP and WDPOMCis2PhP better than their corresponding POMs free porphyrins Cis2PyPTris and Cis2PhPTris, leading to their uses in optical devices.