Synthesis and photophysical properties of new s-triazine derivatives containing A-π-D-π-A quadrupolar branches

Four new s-triazine derivatives containing one or two acceptor-π-donor- π-acceptor quadrupolar branches (S1, E1, S2, E2) were synthesized and characterized by infrared, hydrogen nuclear magnetic resonance, mass spectrometry and elemental analysis. Their photophysical properties were investigated including linear absorption, single-photon excited fluorescence, fluorescence quantum yield, two-photon absorption, and frequency up-converted fluorescence. The spectral positions of the linear absorption and the single-photon excited fluorescence show red-shifts with enhanced electron-accepting ability of the end-group or increasing number of branch. The fluorescence quantum yields of the two-branched compounds (S2, E2) decrease with increasing solvent polarity. Under the excitation of a 800 nm laser with a pulse width of 80 fs, all these compounds emit intense green frequency up-converted fluorescence, and the two-photon absorption cross-sections are 80, 48, 502, and 362 GM for S1, E1, S2, and E2, respectively. This result shows that significant enhancement of the two-photon absorption cross-section can be achieved by sufficient electronic coupling between the strong charge transfer quadrupolar branches through the s-triazine center.