Synthesis and photophysical characterization of substituted Tröger´s bases

DUSSO DIEGO; PRISCILA LANZA CASTRONUOVO; CRISTINA RAMÍREZ; ALEJANDRO PARISE; HERNÁN MONTEJANO; MARIANO VERA; E. LAURA MOYANO; CARLOS CHESTA

Congreso; XIII Elafot.; 2017

Synthetic dyes are generally made up by an electron acceptor (A) and an electron donor (D) group coupled by  or  bridges. The  bridge allows only the adiabatic coupling between A and D, a property that was advantageously used for the design of diodes and photodiodes at the molecular level. [1] We report herein the synthesis and characterization of a series of new chromophores based on a methanodibenzo[b,f][1,5]diazocine bridge (T), whose properties as charge carriers have not been yet explored. These chromophores, also known as Trögers bases (A-T-D), were synthetized as shown below. Trögers 1-4 have a common acceptor (A= CN) and a series of D with increasing electron donor capacity. The brominated precursor of 1-4 were synthetized by reacting 1:1:4 molar mixtures of 4-bromo-2-methylaniline, the corresponding p-D-aniline and p-formaldehyde, respectively, using trifluoroacetic acid (TFA) as solvent. After 24 hours, reactions were quenched with ammonium hydroxide and the obtained precipitates purified by column chromatography to give the products in 30-40 % yields.[2] The microwave-assisted cyanation of the brominated diazocines to yield 1-4 was carried-out using an methodology optimized in our research group. Efficient cyanation was observed using CuCN in dimethyl formamide (DMF) and irradiating with 850W for 30 minutes at 250 °C. After reactant´s conversion was completed, products were extracted with ethyl acetate, dried over MgSO4 and the solvent evaporated under reduced pressure. Yields were excellent to very good: 99 -77 %. Compounds 1-4 were characterized by 1H- and 13C-NMR and IR spectroscopies. The energy of 1-4 absorption (hA) and emission (hE) maxima, fluorescence quantum yields (f) and emission lifetimes (f) were evaluated in a series of aprotic solvent covering a wide range of solvent polarity ( = 2-37). 1-4 show absorption spectra typical of substituted aromatic compounds. 1-3 show a forbidden transition ( ~4000 M-1 cm-1) centred ~ 280-290 nm and a much intense transition at ~ 240 nm. In the case of 4, the forbidden transition is red-shifted (~325 nm). hA slightly shift to lower energies with the increase of solvent polarity. 1-4 are scarcely fluorescent compounds. f, f and hE strongly depend on D and on the solvent polarity. The change of the dipolar moment (µ), associated to the S1S0 transition, were calculated from typical Lippert-Mataga plots. Values of µ = 7.8, 8.3, 9.2 and 12.4 D were obtained for 1-4, respectively. Results suggest that excitation of the Tröger´s base leads to the formation of charge transfer (CT) states with partial (1-3) and full (4) charge separation. Interestingly, the diazocine bridge (T) plays an active role in the CT process, conclusion that is in fully agreement with TD-TFD theoretical calculations performed on the structures of 1-4. [1] R. M. Metzger, Unimolecular Electronics, Chem. Rev, 2015, 115, pp 5056-5115.[2] Q.M. Malik, S. Ijaz, D. C. Craig, A. C. Try. Synthesis and reactivity of dimethoxy-functionalized Tröger´s base analogues. Tetrahedron 2011, 67, pp 5798-5805.