Here we describe the synthesis and characterization of a series of centro-symmetric D-π-A-π-D chromophores based on an s-indacene-1,3,5,7(2H,6H)-tetraone acceptor with conjugated linkers containing thiophene and pyrrole moieties. These materials were studied by UV visible absorption spectroscopy and their third order nonlinear optical properties investigated by z-scan at 1064 nm. The introduction of conjugated pyrrole linkers was found to give an effective third order hyperpolarizability of -1.54 x10-30 esu, and an effective two photon absorption cross section of 6100 GM at 1064 nm. As these chromophores showed comparatively high effective nonlinear absorption they have potential in photonic applications such as optical limiting, optical communication, and for two photon initiated polymerization.
Second-order nonlinear optical (NLO) polymers are emerging materials for next generation photonic and
optoelectronic devices. The synthesis of high figure of merit (μ.β0) chromophores is crucial to the preparation of
such materials, and we have previously reported the facile synthesis of a suite of chromophores with μ.β0 values of
approx. 10,000 x 10-48 esu. These chromophores possess high permanent dipole moments, a property that, when
combined with the high first order polarisability of these molecules, results in high NLO susceptibilities in a
spatially ordered arrays of these compounds. However, their high permanent dipole moment renders them
susceptible to dimerization and aggregation which has the undesirable effect of cancelling the hyperpolarizability
vector associated with these molecules thereby reducing the bulk second order NLO susceptibility.
Theoretical and experimental investigations have determined that the electro optical activity of NLO chromophores
can be maximised through shape modification. The introduction of bulky substituents onto selected chromophore
has been shown to cause the molecular shape to become more spherical (oblate) in nature, and in doing so, this
limits intermolecular interactions. Previous work has focussed on the introduction of bulky substituents in order to
diminish aggregation. We report here on the synthesis of thiazole based zwitterionic chromophores containing bulky
substituents and their spectroscopic properties.
The molecular linear and second-order nonlinear optical (NLO) properties of a series of donor (D)-π-acceptor (A)
merocyanine molecules have been studied in three solvents, dimethylformamide (DMF), tetrahydrofuran (THF), and
chloroform (CHCl3). All the compounds have a cyanodicyanomethylidenedihydrofuran electron acceptor system with either a pyridinylidene or quinolinylidene donor group. In high polarity solvents the molecules with a quinolinylidene donor have
larger first hyperpolarizabilities than those with a pyridinylidene donor, while the opposite is true in low polarity solvents.
The molecules under investigation have an aromatizable donor unit, which leads to a high degree of charge separation in the
ground-state; as a result they have a strong tendency to aggregate. To minimize these interactions arene-rich bulky groups
have been introduced in a number of these compounds.
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