Structure−Property Relationships in Third-Order Nonlinear Optical Chromophores

Abstract

Scientists have sought for over two decades to incorporate the necessary attributes of transparency, stability, and high nonlinear susceptibilities into optimized organic or organometallic chromophores for third-order nonlinear optical (NLO) applications. These investigations have provided an ever-expanding understanding of structure−function relationships for the second hyperpolarizability γ and the bulk third-order nonlinear optical susceptibility χ(3) in organic materials, which are reviewed herein. Contributing to this understanding are the studies of the third-order NLO properties displayed by an array of structurally related organic chromophores based on the conjugated carbon skeletons of hex-3-ene-1,5-diynes (1,2-diethynylethenes, DEEs) and 3,4-diethynylhex-3-ene-1,5-diynes (tetraethynylethenes, TEEs). A comprehensive series of donor (D) and/or acceptor (A) substituted derivatives of DEEs and TEEs has been measured by third-harmonic generation (THG) experiments, and the investigations on these one- and two-dimensionally conjugated chromophores have provided fundamental insight into routes leading to enhanced optical nonlinearities. The molecular characteristics identified to impact the second hyperpolarizability γ include conjugation length, heteroaromatic conjugation, degree of D/A substitution, cis, trans, and geminal D/A conjugation, and molecular asymmetry. A comparison of NLO properties for small molecular systems to those of a number of larger oligomers based on the DEE and TEE framework is also presented.

Conjugation and optoelectronic properties of acetylenic scaffolds and charge-transfer chromophores

“…The synthesis of a new polymer bearing lateral donor-acceptor (D-A)-substituted DEE chromophores was accomplished by ring-opening metathesis polymerization (ROMP). The soluble polymer with an average of 40 monomeric units forms good optical-quality films, with the third-order susceptibility χ (3) at a wavelength of 1907 nm being 100 times greater than that of fused silica. After electrical poling, the polymer displayed a high second-order nonlinear optical (NLO) coefficient (d 33 ) [21].…”

“…iv. For an improved synthesis of the recently described expanded octamethoxycubane 7 with a central C 56 core [34], formally obtained by inserting buta-1,3-diyne-1,4-diyl moieties into all C(sp 3 )-C(sp 3 ) bonds of octamethoxycubane [35], the preparation of optically pure trialkynylmethane building blocks as corner modules was required. Remarkably, the chirality of these compounds derives only from differential alkyne protection.…”

“…It is well known that the molecular first-order hyperpolarizability (β) depends not only on the strength of the donor and acceptor groups, but also on the nature of the π-conjugated spacer through which they interact. While well-defined guidelines to enhance β (and the bulk second-order susceptibility χ (2) ) have been established, definite principles for achieving high molecular second-order hyperpolarizability (γ) and high-bulk third-order susceptibility (χ (3) ) are still being pursued [3,69].…”

“…1) with strong electron donors, such as DMA, and acceptors, such as 4-nitrophenyl moieties [3,12,70]. Some of these compounds were shown to feature high second-and third-order optical nonlinearities.…”

“…Highly conjugated, carbon-rich organic molecules featuring tunable structural and optoelectronic properties have been recognized as promising candidates for use in next-generation electronic and optoelectronic devices [1][2][3][4][5][6]. Twenty years ago, in 1987, we started a research program in acetylene chemistry with the aim to synthesize acetylenic molecular carbon allotropes.…”