Resonant Nonlinear Absorption in Zn-Phthalocyanines

Abstract

In this work, we investigate the nonlinear absorption dynamics of Zn phthalocyanine in dimethyl sulfoxide (DMSO). We used single pulse and pulse train Z-scan techniques to determine the dynamics and absorption cross-sections of singlet and triplet states at 532 nm. The excited singlet state absorption cross-section was determined to be 3.2 times higher than the ground state one, giving rise to reverse saturable absorption. We also observed that reverse saturable absorption occurs from the triplet state, after its population by intersystem crossing, whose characteristic time was determined to be 8.9 ns. The triplet state absorption cross-section determined is 2.6 times higher than the ground state one. In addition, we used the white light continuum Z-scan to evaluate the singlet excited state spectrum from 450 to 710 nm. The results show two well-defined regions, one above 600 nm, where reverse saturable absorption is predominant. Below 600 nm, we detected a strong saturable absorption. A three-energy-level diagram was used to explain the experimental results, leading to the excited state absorption cross-section determination from 450 nm up to 710 nm.

Mechanism of the Zn(II)Phthalocyanines’ Photochemical Reactions Depending on the Number of Substituents and Geometry

“…Such lifetimes are in agreement with the ones reported for ZnPcs dissolved in chloroform [16] and are close to the one reported for B4 dissolved in THF [17]. Once the fluorescence lifetime and quantum yields are determined, one could estimate the radiative rate = ϕ / from all samples dissolved in chloroform.…”

“…This quantity will be needed to describe the population dynamics observed in ZnPcs excited states employing a rate equation model. The fluorescence spectra displays a single emission band that ends at approximately 800 nm, a behavior similar to the ones already reported in the literature [16,17]. …”

“…A decrease in the NT only occurs if the excited state has an absorption cross-section higher than the one of the ground state. A similar behavior of the decrease of the NT at 532 nm was also reported for symmetric ZnPc with only hydrogen as the peripheral groups [16]. The value obtained throughout the fittings for the singlet excited state absorption cross-sections were σ 1n " 3.1˘0.4ˆ10´1 7 cm 2 for B4 (2.8-times higher than the ground state absorption cross-section), σ 1n " 2.4˘0.2ˆ10´1 7 cm 2 for AB3 (3.2-times higher than the ground state absorption cross-section) and σ 1n " 1.9˘0.4ˆ10´1 7 cm 2 for ABAB (four-times higher than the ground state absorption cross-section).…”

“…In this case, the triplet states were neglected because the single and WLC pulses have durations of about 100 ps and 4 ps, respectively. During these times, which are much shorter than the intersystem crossing time (in the order of ns for ZnPc [16]), the population fraction transferred for the triplet (n T1 ptq) is negligible, simplifying Equation (1) for only singlet terms. Thus, the rate equations necessary to describe the population dynamics in the three singlet energy levels are:…”

“…Zinc phthalocyanines [16,17] present interesting fluorescence properties, high triplet yield, long lifetime and optical nonlinearities [18][19][20][21], such as large nonlinear absorption due to the intense reverse saturable absorption [22] (RSA), which led us to design 2,3,9,10,16,17,23,24-octakis(hexylthio)phthalocyaninato)zinc (B4), (2,3,9,10,16,17 hexakis(hexylthio)phthalocyaninato)zinc Herein, the synthesis and characterization of B4, AB3 and ABAB are reported, as well as their photophysical and nonlinear optical properties. For the analysis of optical nonlinearities dynamics and excited states' properties, single pulse [23] and pulse trains [24] Z-scan techniques were used, both at 532 nm.…”