The Design and Spectroscopy of Aminosalicylate Molecules to Exhibit Dual and Triple Fluorescence.

Three comparative fluorescence emissions are shown to be observable from a common initial excited singlet state in a prototype aminosalicylate molecule: p-NR _2C_6H_3 (OR^')COOR ^{''}, PDASE (R = CH_3, R^' = H and R^{'' } = CH_3). All molecular structures are with the (OR^' ) group ortho to the COOR^{ ''} group. The steady -state fluorescences are assigned as the normal fluorescence {rm S_1(pi,pi^ *) to S_0}, the twisted intramolecular charge-transfer or TICT-state fluorescence S_sp{1}{'' }(TICT) to S _0, and the intramolecular proton-transfer fluorescence S_sp{1}{' }(PT) to S_sp {0}{'}. Two excited state absorptions are also believed to be present for PDASE. The excited-state absorptions are assigned as the absorption from the first excited TICT singlet S_sp {n}{''}(TICT) >=ts S_sp{1} {''}(TICT), and the absorption from the first excited state of the proton-transfer tautomer S_sp{n}{' }(PT) >=ts S _sp{1}{'}(PT). Several compounds were designed and synthesized to isolate the emission-type that was of interest. Among these were MABAE (R = H, R^' = CH_3 and R^{ ''} = CH_3 ) which isolates for normal emission processes; PASE (R = H, R^' = H and R^{''} = CH_3) which isolates for proton -transfer processes in the shifted or F_2 band; and, MDABAE (R = CH_3, R^' = CH_3 and R^{'' } = CH_3) which isolates for TICT processes in the F_2 band. Using steady-state absorption and emission, the relative shift in fluorescence wavelength maxima vs. the solvent polarity index E_{rm T}, picosecond optical multichannel analyzer resolved transient absorption, and picosecond streak camera resolved fluorescence dynamics the assignments for the studied molecular systems were corroborated.