Nonlinear Optical Pigments. Two-Photon Absorption in Crosslinked Conjugated Polymers and Prospects for Remote Nonlinear Optical Thermometry.

Jan K Zaręba,Marcin Nyk,Marek Samoć

doi:10.3390/polym12081670

Abstract

Nonlinear optical (NLO) pigments are compounds insoluble in solvents that exhibit phenomena related to nonlinear optical susceptibilities (χ(n) where n = 2,3,...), e.g., two-photon absorption (2PA) which is related to the imaginary part of χ(3). Determination of spectrally-resolved 2PA properties for NLO pigments of macromolecular nature, such as coordination polymers or crosslinked polymers, has long been a challenging issue due to their particulate form, precluding characterizations with standard techniques such as Z-scan. In this contribution, we investigate thus far unknown spectrally-resolved 2PA properties of a new subclass of NLO pigments—crosslinked conjugated polymers. The studied compounds are built up from electron-donating (triphenylamine) and electron-withdrawing (2,2’-bipyridine) structural fragments joined by vinylene (Pol1) or vinyl(4-ethynylphenyl) (Pol2) aromatic bridges. 2PA properties of these polymers have been characterized in broad spectral range by specially modified two-photon excited fluorescence (TPEF) techniques: solid state TPEF (SSTPEF) and internal standard TPEF (ISTPEF). The impact of self-aggregation of aromatic backbones on the 2PA properties of the polymers has been evaluated through extended comparisons of NLO parameters, i.e., 2PA cross sections (σ2) and molar-mass normalized 2PA merit factors (σ2/M) with those of small-molecular model compounds: Mod1 and Mod2. By doing this, we found that the 2PA response of Pol1 and Pol2 is improved 2–3 times versus respective model compounds in the solid state form. Further comparisons with 2PA results collected for diluted solutions of Mod1 and Mod2 supports the notion that self-aggregated structure contributes to the observed enhancement of 2PA response. On the other hand, it is clear that Pol1 and Pol2 suffer from aggregation-caused quenching phenomenon, well reflected in time-resolved fluorescence properties as well as in relatively low values of quantum yield of fluorescence. Accordingly, despite improved intrinsic 2PA response, the effective intensity of two-photon excited emission for Pol1 and Pol2 is slightly lower relative to Mod1 and Mod2. Finally, we explore temperature-resolved luminescence properties under one- (377 nm), two- (820 nm), and three-photon excitation (1020 nm) conditions of postsynthetically Eu3+-functionalized material, Pol1-Eu, and discuss its suitability for temperature sensing applications.

Highlights

The interest in polymers as candidate materials for applications employing second-order and third-order nonlinear optical (NLO) phenomena dates back to the 1980s and has been maintained ever since [1,2]
Since the determination of 2PA cross sections of NLO pigments employed in this work relies on the fluorescence-based techniques (SSTPEF and internal standard TPEF (ISTPEF)), the target material, to be of any use for such studies, must provide luminescence output that can be excited under two-photon conditions in a wide range of excitation wavelengths; in other words, the NLO pigment must be an NLO luminophore (NLO phosphor)
Some indication on how the NLO-induced luminescence can be harnessed for external stimuli sensing has been presented in our previous contribution, in which we demonstrated that the other representatives of NLO pigments—coordination polymers (CPs) built from lanthanide ions (Gd3+ as matrix and Eu3+, Tb3+ ions as dopants), 1,3,5-benzenetricarboxylic acid and phenantroline—can serve as NLO luminescent thermometers [48]

Summary

Introduction

The interest in polymers as candidate materials for applications employing second-order and third-order nonlinear optical (NLO) phenomena dates back to the 1980s and has been maintained ever since [1,2]. This has been elicited by the promising optical and structural properties of polymeric. Polymers 2020, 12, 1670 materials, as well as their ease of modification, largely defined by their organic nature. NLO chromophores [11,12,13,14] These principles are commonplace tools that assist the design of new small-molecular and polymeric compounds for nonlinear optics. From the processing point of view, polymers and polymeric materials appear to be exceptionally versatile as well, as some of them can be prepared in the form of liquid solutions or dispersions and can be readily processed as powders, fibers, and films, and some selected examples of them can be obtained as single crystals

Methods

Results

Conclusion