Diquat Derivatives: Highly Active, Two-Dimensional Nonlinear Optical Chromophores with Potential Redox Switchability

Abstract

In this article, we present a detailed study of structure−activity relationships in diquaternized 2,2′-bipyridyl (diquat) derivatives. Sixteen new chromophores have been synthesized, with variations in the amino electron donor substituents, π-conjugated bridge, and alkyl diquaternizing unit. Our aim is to combine very large, two-dimensional (2D) quadratic nonlinear optical (NLO) responses with reversible redox chemistry. The chromophores have been characterized as their PF6 − salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Their visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all show two reversible diquat-based reductions. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering with an 800 nm laser, and Stark spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β0. The directly and indirectly derived β values are large and increase with the extent of π-conjugation and electron donor strength. Extending the quaternizing alkyl linkage always increases the ICT energy and decreases the E 1/2 values for diquat reduction, but a compensating increase in the ICT intensity prevents significant decreases in Stark-based β0 responses. Nine single-crystal X-ray structures have also been obtained. Time-dependent density functional theory clarifies the molecular electronic/optical properties, and finite field calculations agree with polarized HRS data in that the NLO responses of the disubstituted species are dominated by ‘off-diagonal’ β zyy components. The most significant findings of these studies are: (i) β0 values as much as 6 times that of the chromophore in the technologically important material (E)-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate; (ii) reversible electrochemistry that offers potential for redox-switching of optical properties over multiple states; (iii) strongly 2D NLO responses that may be exploited for novel practical applications; (iv) a new polar material, suitable for bulk NLO behavior.

Spectroelectrochemistry of alternating ambipolar copolymers of 4,4′- and 2,2′-bipyridine isomers and quaterthiophene

“…Likewise for 1 and 2, oxidation of 1a(PF 6 ) 2 , 1a(OTf) 2 , and 2a(PF 6 ) 2 is irreversible and results in electroactive film deposition. In the range of negative potentials, 1a(PF 6 ) 2 , 1a(OTf) 2 , and 2a(PF 6 ) 2 demonstrate two step reduction process characteristic for viologens [17] and diquats [73]. The course of the redox reactions turns out to be strongly counterion dependent and Fig.…”

Heterometallic transition metal clusters and cluster-supported coordination polymers derived from Tp- and Tp*-based Mo(W) sulfido precursors

“…Nano-composites, organic dyes and functional coordination compounds exhibit promising non-linear optical properties [107][108][109][110][111][112]. Mo(W)-Cu(Ag)-S clusters or cluster-based coordination polymers have proven useful in the last two decades due to their extensive d-p delocalized system and d-d conjugated system [81,113,114].…”

Multimodal Metal Cation Sensing with Bis(macrocyclic) Dye

“…The parent dyes M1 and M2 and bisA C H T U N G T R E N N U N G (styryl) dye 1 resemble stilbazolium species, with lowest-energy electronic transitions of HOMO!LUMO character. [18] The HOMO and HOMO-1 are always mostly located on the electronrich phenylazathia-15-crown-5 or benzo-15-crown-5 groups, but with significant contributions from the ethenyl bridges and the pyridinium unit. The LUMO, LUMO + 1, and LUMO + 2 are concentrated on the electron-accepting pyridinium fragment, with the LUMO + 2 having especially limited contributions from the rest of the molecule.…”