PHOTO-ROTAMERISM OF 2,2'-PYRIDIL IN DIFFERENT ENVIRONMENTS - A QUEST FOR GEOMETRY IN EXCITED-STATES

Temperature and solvent (polarity) dependent photorotamerism in 2,2'-p yridil (P) has been observed from photoluminescence studies. Dual fluo rescence and phosphorescence of P also substantiate the idea of the pr esence of different rotamers. The changes in intensities and positions of the two fluorescence peaks of P in aqueous beta-cyclodextrin (beta -CD) solution are due to the impediment in intramolecular rotation and the stifling of the radiationless process caused by partial inclusion in the beta-CD cavity. At high concentration, the T <-- S-0 transitio n could be observed at 300 K. Two close-lying, strongly coupled (3) pi ,pi (T-2) and (3)n,pi* (T-1) states of P eventuate in bi-exponential triplet decay. Protonation is favoured at the site of nitrogen.