ENERGY-TRANSFER IN THE INVERTED REGION - CALCULATION OF RELATIVE RATE CONSTANTS BY EMISSION SPECTRAL FITTING

Bimolecular energy transfer quenching of the metal-to-ligand-charge-transfer (MLCT) excited states of a series of 2,2'-bipyridine (bpy) complexes of Os-II by anthracene or 2,3-benzanthracene (tetracene) has been studied in 3:1 (v/v) acetonitrile-benzene at room temperature. Energy transfer rate constants (k(q)) vary from 6.3 x 10(7) to 5.1 x 10(9) M(-1) s(-1), which is below the diffusion-controlled limit of 9.1 x 10(9) M(-1) s(-1). For anthracene and the first triplet state (T-1) of 2,3-benzanthracene, kg reaches a maximum at a driving force (-Delta.G degrees) of +0.52 eV. With a further increase in -Delta C degrees, k(q) decreases, falling as low as 3.2 x 10(8) M(-1) s(-1) with 2.3-benzanthracene as quencher, consistent with inverted behavior. Energy transfer occurs via the triplet-triplet exchange (Dexter) mechanism. There is evidence for quenching by the second tripler state of 2,3-benzanthracene (T-2) at even higher driving force. Application of a Franck-Condon analysis to the emission spectral profiles of the complexes and phosphorescence from anthracene has provided kinetic parameters for calculating relative energy transfer rate constants based on the usual Golden Rule formalism. When combined with an appropriate quenching model (including T-2 for 2,3-benzanthracene), it is possible to account for the driving force dependence of k(q) quantitatively, thus demonstrating the use of spectral fitting parameters to calculate relative rate constants for energy transfer. On the basis of this analysis and comparison between experimental and calculated k(q) values, V-1 = 2.5 cm(-1) for quenching by anthracene and T-1 of 2,3-benzanthracene, and V-2 = 8 cm(-1) for quenching by T-2 of 2,3-benzanthracene.