Tuning the optical properties of NLO push-pull chromophores through halogen bonding.

Among linear -conjugated organic molecules displaying strong second order nonlinear optical (NLO) and emissive properties, p-phenylene vinylene (PPV) derivatives are of particular interest, due to their broad absorption bands and high luminescence quantum yields.[1] Moreover, they can be used as convenient -bridges between electron donor and acceptor groups, leading to push-pull chromophores with tunable optical behavior. In the search of noncovalent interactions for driving the self-assembly of such derivatives, halogen bonding (XB) represents a rather new and effective tool. According to our previous results, XB can play a key-role in tuning NLO properties of self-complementary tectons in solution.[2] Two new push-pull PPV derivatives (Figure 1), bearing a N,N-dimethylamino group as the electron donor moiety and a tetrafluorophenyl ring (either p-iodinated or not) as the electron acceptor site, were synthesized. In the case of iodinated derivative 1a, the two end units can act as self-complementary XB acceptor and donor sites, driving its self-assembly into head-to-tail halogen-bonded infinite chains.[3] For compound 1b, which is devoided of the I atom, aryl-fluoroaryl interactions are the only driving forces responsible of its crystal packing.[4] We present herein a comparative studies on solution NLO and solid-state emissive properties of these two chromophores, focusing on the influence exerted by XB on their NLO properties in solution and solid-state photoluminescence.