Synthesis and photophysical characterization of substituted Tröger´s bases.

DIEGO DUSSO; PRISCILA LANZA CASTRONUOVO; RAMIREZ, CRISTINA L.; ALEJANDRO PARISSE; HERNAN MONTEJANO; D. MARIANO VERA; ELIZABETH L. MOYANO; CARLOS CHESTA

Villa Carlos Paz

Congreso; XIII ELAFOT; 2017

Synthetic dyes are generally made up by an electron acceptor (A) and an electron donor (D) groupcoupled by  or  bridges. The  bridge allows only the adiabatic coupling between A and D, aproperty that was advantageously used for the design of diodes and photodiodes at the molecularlevel. [1] We report herein the synthesis and characterization of a series of new chromophores based ona methanodibenzo[b,f][1,5]diazocine bridge (T), whose properties as charge carriers have not beenyet explored. These chromophores, also known as Trögers bases (A-T-D), were synthetized as shownbelow. Trögers 1-4 have a common acceptor (A= CN) and a series of D with increasing electrondonor 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 trifluoroaceticacid (TFA) as solvent. After 24 hours, reactions were quenched with ammonium hydroxide and theobtained 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 anmethodology optimized in our research group. Efficient cyanation was observed using CuCN indimethyl formamide (DMF) and irradiating with 850W for 30 minutes at 250 °C. After reactant´sconversion was completed, products were extracted with ethyl acetate, dried over MgSO4 and thesolvent 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) andemission lifetimes ( f) were evaluated in a series of aprotic solvent covering a wide range of solventpolarity ( = 2-37). 1-4 show absorption spectra typical of substituted aromatic compounds. 1-3 showa forbidden transition ( ~4000 M-1 cm-1) centred ~ 280-290 nm and a much intense transition at ~ 240nm. In the case of 4, the forbidden transition is red-shifted (~325 nm). hA slightly shift to lowerenergies with the increase of solvent polarity. 1-4 are scarcely fluorescent compounds. f, fand hEstrongly depend on D and on the solvent polarity. The change of the dipolar moment (μ), associatedto 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´sbase leads to the formation of charge transfer (CT) states with partial (1-3) and full (4) chargeseparation. Interestingly, the diazocine bridge (T) plays an active role in the CT process, conclusionthat is in fully agreement with TD-TFD theoretical calculations performed on the structures of 1-4.