Photophysical characterisation of Tröger´s base derivatives.

LEANDRO TRUPP; BARJA, BEATRIZ C.; MARIANA VARDÉ; BRUTTOMESSO, ANDREA C.

Villa Carlos Paz, Córdoba

Encuentro; XIII ELAFOT, Latin American Meeting on Photochemistry and Photobiology.; 2017

The interest in Tröger´s base analgues (TBs) emerged because the two aromatic rings fused to the central bicyclic framework are nearly perpendicular to each other, creating a rigid, V-shaped molecular scaffold with a distance of ca. 1 nm between the two extremities (Figure 1). In addition, TBs are chiral and fluorescent amines. Their chirality results from the very high energy barrier for the inversion of the nitrogen atoms and the fluorescence from the rigidity of the molecules. Due to these unique characteristics, Tröger?s base derivates have countless potential applications, such as molecular recognition, catalysis, supramolecular chemistry and new materials among others [1]. Figure 2In the present work, four new Tröger?s base derivatives 1?4 (Figure 2) were synthetised in one to four steps, and their structures were confirmed by 1D and 2D NMR spectroscopy and mass spectrometry. Their stationary and dynamic photophysical properties were studied in hexane, dichloromethane (DCM), acetonitrile (MeCN) and ethanol (EtOH). Figure 3UV-Vis absorption spectra show bands centered in the 250?350 nm region for all compounds in every solvent, with high absorption coeficients of around 103?105 M?1cm?1, which are consistent with spin and simmetry allowed * transitions. All compounds emit in the 260?500 nm region and differences in the position and shape of the emission bands are observed, being strongly dependent both on the substituent and the solvent. These differences are most significant in DCM (Figure 3). The fluorescence decay profiles in hexane and EtOH show a biexponential time decay with lifetimes of approx. 1?2 ns and 5?7 ns. These results differ from those obtained in MeCN and DCM, where the decay profiles showed mono or biexponential behaviors depending on the substituent. In all cases, fluorescence lifetimes are consistent with * transitions. The largest solvatochromism is observed in 1 (Figure 4). This bathochromic effect indicates that the dipole moment is significantly larger in the excited state than in the ground state, which could be related to the extention of the aromatic conjugation to the isonitrile moiety. Specific solvent effects (i.e. hydrogen bonding) were observed for ethanol from the Lippert plot, whereas this behavior was not evident in other compounds.As a continuation of this work, time-resolved emission spectroscopy will be performed to analyze the photophysical reaction mechanisms and the excited state decays involved. Despite Tröger?s bases interesting geometrical properties, almost no information regarding the photophysics and electronic structure of these analogues was reported [2] and this study represents one of the first approaches in this area. [1] S. Sergeyev, Helvetica Chimica Acta 2009, 92, 415.[2] D.M. Pimentel Aroche, J.M. Toledo, R. Roceti Descalzo, P.F.B. Gonçalves, F.S. Rodembush, New J. Chem. 2015, 39, 6987.