Solvent-controlled excited state behavior: 2-(2'-Pyridyl)indoles in alcohols

Efficient fluorescence quenching caused by rapid internal conversion from the first excited singlet state is observed in alcohol solutions of 2-(2'-pyridy1)indoles, molecules which may simultaneously act as hydrogen bonding donors and accepters. Electronic and infrared absorption studies show that upon adding alcohols to nonpolar solutions of pyridylindoles, 1:1 complexes are formed in the ground state, with hydrogen bonding occurring to the indole NH group. At higher alcohol concentrations 1:n (n greater than or equal to 2) solvates dominate. Investigations of the fluorescence intensity as a function of temperature and deuterium substitution in the hydroxylic group of the alcohol, and the results obtained for the N-methylated derivatives and 2-phenylindole show that the quenching may be described by a stepwise mechanism. First, a 1:1 cyclic, doubly hydrogen-bonded complex between alcohol and pyridylindole is formed after photoexcitation. This process is controlled by solvent reorientation. Excited state double proton transfer in such a complex opens up a fast internal conversion channel. The driving force for phototautomerization arises as a result of the excited state increase of the basicity of the pyridine nitrogen atom and the enhanced acidity of the indole NH group. This effect was also predicted by calculations of excited state valence electron potentials, parameters that can be used as acido-basic reactivity indices.