Novel tripodal aminoisoquinolyl-based sensors designed for selective Zn detection and luminescence approaching the visible spectrum

INOR 34

Roy P. Planalp, roy.planalp@unh.edu1, Daniel P. Kennedy, dpk2@unh.edu1, Antonio G. DiPasquale2, and Arnold L. Rheingold2. (1) Department of Chemistry, University of New Hampshire, Durham, NH 03824, (2) Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093

We report novel molecules based in the ‘TAME' (TAME = 1,1,1-tris(aminomethyl)ethane) framework with aminoisoquinoline groups which detect Zn(II) using coordination-enhanced fluorescence (CEF) and photoinduced electron transfer (PET).  The TAME scaffold is chosen for selective Zn(II) detection over Cd(II), because it provides rigid coordination of smaller di- or trivalent metal ions such as Zn(II) or Ga(III), vs. larger ones such as In(III).  Appending electron-releasing groups on the isoquinoline chromophore gives lower-energy emission.  The synthesis of 1 (TAMEisoquin) and 2 (6,7-DMTI, where 6,7-DMTI=6,7-dimethoxyTAMEisoquin) and structure of [Zn-1](ClO4)2 will be described.  Photophysical studies indicate modest luminescent effects in Zn(1)]2+, namely a 15-fold enhancement of fluorescence vs. unbound 1 and quantum yield ΦZn-1 = 0.77%.  Notably, the response of 1 to Zn(II) is selective in that none of the metal ions Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Cd(II), Na(I), K(I), Ca(II) and Ag(I) show greater than a ± 1-fold effect on the luminescence intensity of 1.  This finding is attributed to the match of donor properties and cavity size of 1 to Zn(II).  The 6,7-dimethoxyisoquinoline chromophore was synthesized from the appropriately substituted 3,4-dimethoxybenzaldehyde in six steps giving 6,7-dimethoxy-1-isoquinolinecarboxaldehyde in 11.5% overall yield, unoptimized.  The aldehyde was then appended to the TAME framework via a Ni(II) mediated template reaction followed by NaBH4 reduction and liberation of 6,7-DMTI (2) in 26.7% yield.  Photophysical studies revealed that in the presence of Zn(II) 2 emits in the UV (λex=326nm, λem=360nm) but possesses an enhanced quantum yield ΦZn-2 = 12.7% as compared to Zn-1.  Likewise, the response of 2 to Zn(II) is selective in that none of the metal ions Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Cd(II), Na(I), K(I), Ca(II) and Ag(I) show greater than a ± 2-fold effect on the luminescence intensity of 2.