Excited State Chemistry of the Ferrocyanide Ion in Aqueous Solution. II. Photoaquation

Seven‐nsec laser pulses at 337 nm from an N₂ gas laser and continuous illumination at 254, 313, and 365 nm were used to investigate the formation of (Fe(CN)₅H₂O)³⁻ from photoexcited states of ${\mathrm{Fe(CN)}}_6^{\text{4−}}$ and its protonated form in aqueous solution. The same pentacyano‐aquo complex is formed at all wave‐lengths. It is formed in $\text{≤ 1\hspace{0.17em}}\mathrm{nsec}$ as shown by the laser results. The quantum yield is constant at 337 nm between pH 3.8–10.5. The spectrum of the product changes and the quantum yield appears to decrease at lower pH. The quantum yield at pH 9 is the same at 365 and 313 nm and decreases to half at 254 nm. Photoaquation occurs via the lowest singlet ¹T_1g state. Excitation to higher singlet states results in competition between decay to the lowest singlet and transition from the short‐lived higher singlets to a CTTS state yielding the hydrated electron.