Reductive photocyanation of 1,10-phenanthroline in aqueous media

Reductive photocyanation of 1,10-phenanthroline (phen) took place in an aqueous NaCN solution, yielding 5-cyano-5,6-dihydro-1,10-phenanthroline (Hphen-CN) with the reaction and quantum yields of 5–50% and 10−3 to 10−4, respectively. The photoreaction in the presence of Mg2+ (aerated conditions) improved the reaction quantum yield by a factor of ∼7 as compared to that without Mg2+, through coordination of Mg2+ with phen: phen:Mg2+ complex. The fluorescence lifetimes of phen and the phen:Mg2+ complex were ∼3 and ∼10 ns, respectively, and the fluorescence of the phen:Mg2+ complex was shown to be quenched more efficiently by a cyanide ion as compared to that of phen itself. The fluorescence quenching percents of phen by NaCN in the absence and presence of Mg2+ were 3 and 24%, respectively, and the eight-fold increase in the quenching percent in the presence of Mg2+ gave rise to the approximately seven-fold increase in the reaction quantum yield. On the basis of such results, we proposed that the present photocyanation reaction proceeded via electron transfer from a cyanide ion to the excited singlet-states of phen and/or the phen:Mg2+ complex as the primary step: anion radical (SN-R1Ar*) mechanism. The characteristic features of the present photoreaction of phen in the presence of Mg2+ and the possible reaction mechanisms are discussed in detail.