Novel Ferrocene‐Appended β‐Ketoimines and Related BF<sub>2</sub> Derivatives with Significant Aggregation‐Induced Emission and Second‐Order Nonlinear Optical Properties
Ezhumalai David, Alessia Colombo, Claudia Dragonetti, Nallasamy Palanisami
Abstract
Abstract A series of new β‐ketoimines containing a ferrocene moiety and their BF 2 complexes have been synthesized and structurally characterized. The solvatochromism of the β‐ketoimines was studied, putting in evidence a redshift with increasing solvent polarity. This positive solvatochromism can be attributed to a more polarized excited state compared with the ground state, due to intramolecular charge transfer (ICT) transitions. The β‐ketoimines exhibited weak emission, attributable to the excited‐state intramolecular proton transfer (ESIPT) phenomenon. This ESIPT effect is suppressed upon restriction of the keto‐enamine tautomerism, induced upon addition of BF 3 ⋅ OEt 2 , which afforded the related BF 2 complexes, characterized by an enhancement of the fluorescence through the ICT effect. Both the β‐ketoimines and BF 2 complexes exhibited significant aggregation‐induced emission behavior in mixtures of CH 3 CN/H 2 O, due to restriction of intramolecular rotation in the aggregated state. The frontier molecular orbital levels, ground‐ and excited‐state dipole moments ( μ g and μ e ), and the origin of electronic absorption spectra were studied by time‐dependent DFT calculations. The second‐order nonlinear optical (NLO) properties were determined by the electric‐field‐induced second‐harmonic generation technique. The μβ 1907 values of the β‐ketoimines increased upon the formation of the related BF 2 complexes, mainly due to an enhancement of the ground‐state dipole moment. The results presented here reveal that some of these novel compounds are excellent multifunctional candidates for NLO and luminescence applications.