Synthesis And Photoproperties Of A Quencher, Na[Cr(1,3-Diaminopropane)(Cn)(4)], And Reinvestigation Of Cr(1,3-Diaminopropane)(3)(3+) Photoproduct Quenching

The photochemical, photophysical, and general properties of the compound Na[Cr(tn)(CN)(4)] (tn = 1,3-diaminopropane) suggest that it has considerable potential as a new and useful quencher. It does not absorb over most of the visible spectrum, and it is reasonably stable both to thermo- and photosubstitution processes. Only at a pH of 2 or less does acid-catalyzed loss of cyanide become significant; the first-order rate constant at pH 2 and 20 degrees C was 4 x 10(-5) s(-1), for a half-life of 5 h. There was a quantum yield of 0.040 +/- 0.006 for photoaquation on irradiation at 406 nm. In dmso or dmf room-temperature solutions, the compound emits at 748 nm; in these solvents this emission can be used as a probe for energy transfer to the quencher. At the same time, its low charge reduces the danger of precipitation of quenching partner cations as double salts or the formation of ion pairs. The efficiency of this quencher was explored through a comparison of its quenching of Cr(tn)(3)(3+) with that by hydroxide; the quenching rate constants at 20 degrees C were (6 +/- 2) x 10(8) and 6 x 10(10) M(-1) s(-1), respectively. This quenching study confirmed an earlier report of different photoproduct isomer ratios for the quenched and unquenched photochemistry of Cr(tn)(3)(3+) but did not show the wavelength effect reported earlier.