| Abstract | The fast relaxation dynamics of 1,6-dihydro-2-methyl-6-oxo-3,4¢-bipyridine-5-carbonitrile (milrinone, MIR),
a cardiotonic drug, has been characterized in water solutions at different pH. In acidic media, a blue emission
reflects a charge-transfer state in the cation (C) leading to a more stabilized structure with an emission
lifetime of 90 ps. The emission lifetimes of the keto (K) and anion (A) structures are 65 and 310 ps,
respectively. Reasons for efficient nonradiative channels are discussed in terms of hydrogen-bonding
interactions, intramolecular charge transfer (ICT), and twisting motion. A blue nanosecond-emission observed
in almost all the studied pH range is suggested to be due to relaxed K due to an ICT reaction. B3LYP
(6-31+G**) calculations showed that, in a water cavity, K is more stable than the enol form by 7 kcal/mol,
and the ICT may take place within the pyridone moiety. At the physiological pH, the inotropic K structure
is the dominant species (100%). |