COPPER(I) COMPLEXES OF 1,10-PHENANTHROLINE DERIVATIVES: PROPERTIES OF THE CHARGE-TRANSFER PHOTOEXCITED STATES
Abstract
The photophysical properties of a series of complexes of the type Cu(NN)(,2)('+) have been studied where NN is one of the ligands (2,9-dimethyl-1,10-phenanthroline, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline or 4,4',6,6'-tetramethyl-2,2'-bipyridine). The quantum yields for these complexes in CH(,2)Cl(,2) at 25(DEGREES)C are 2.1 x 10('-4), 2.5 x 10('-4) and 0.5 x 10('-4) respectively. It has been found that the emission intensity of these complexes decreases as the temperature is lowered. These results have been interpreted in terms of two emitting states where the population of the upper level, which has a more favorable radiative rate constant, requires thermal energy. In the case of the 2,9-dimethyl-1,10-phenanthroline complex, the radiative rate constant for the lowest energy emitting state was determined to be on the order of 10('3) s('-1), typical of a triplet state, while the higher energy state exhibited a radiative rate constant of 10('7) s('-1), typical of a singlet state. The energy separation of these two states is approximately 1800 cm('-1). The kinetics of quenching the ('3)d-(pi)* excited state of Cu(dpp)(,2)('+) (where dpp is 2,9-diphenyl-1,10-phenanthroline) by a series of Cr(acac)(,3) derivatives (acac is 2,4-pentanedionate) and a series of nitroaromatic compounds in CH(,2)Cl(,2) at 20(DEGREES)C has also been studied. For the nitroaromatic compounds, the quenching rate falls off as the electrode potential of the quencher ranges below that of *Cu(dpp)(,2)('+). These results are consistent with an electron transfer mechanism, and the results were analyzed by a variation of Marcus theory. The self-exchange rate has been estimated at 8 x 10('9) M('-1) s('-1). The Cr(III) quenchers span a wider potential range and are also capable of undergoing energy-transfer quenching to low-lying doublet states. The rates show a sigmoidal dependence on quencher potential indicating a shift from an electron-transfer to an energy-transfer quenching mechanism. The self-exchange rate for the Cr(III)/Cr(II) couple is estimated to be 2 x 10('9) M('-1) s('-1), suggesting that the Cr(II) forms are low-spin.
Degree
Ph.D.
Subject Area
Chemistry
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