GROUND AND EXCITED-STATE PROTOTROPIC BEHAVIOR OF 1-(PURIN-6-YL)-3-METHYLIMIDAZOLIUM CHLORIDE

The ground and excited state acid-base properties of the compound 1-(p urin-6-yl)-3-methylimidazolium chloride, 2, have been studied in aqueo us solutions by means of UV absorption and fluorescence (steady state and time resolved) measurements. The ground state exhibits cation-zwit terion and cation-divalent cation equilibria with pK(a) = 7.25 +/- 0.0 5 and -1.3 +/- 0.1, respectively. Within the pH range 5.3-3, the catio nic form of 2 undergoes deprotonation in the excited singlet state lea ding to a highly fluorescent zwitterionic species. However, the equili brium is not established within the excited singlet state lifetime and it is estimated that only a fraction (approximate to 12%) of the exci ted molecules undergoes deprotonation. Using the Forster cycle, an exc ited singlet state pK(a) = 1.5 is estimated from the steady state abs orption and fluorescence emission spectral data. In the range 1 < pH < 3.5, phototautomerization of the cation, leading to an excited tautom er protonated N3, occurs in addition to deprotonation. At high aciditi es (pH < 1) only the cation and the phototautomer emissions are observ ed. The fluorescence parameters for both the cation and phototautomer correlate well with the fluorescence data for the respective methyl an alogs of the two compounds, namely 1-(9-methylpurin-6-yl)-3-methylimid azolium chloride, 3, and 1-(3-methylpurin-6-yl)-3-methylimidazolium ch loride, 4. (C) 1997 Elsevier Science S.A.