Photophysics of diplatinum polyynediyl oligomers: chain length dependence of the triplet state in sp carbon chains.

The series of polyynes with the structure trans, trans-[Ar-Pt(P 2)-(C[triple bond]C) n -Pt(P 2)-Ar], where P = tri( p-tolyl)phosphine, Ar = p-tolyl, and n = 3, 4, 5, 6 (6, 8, 10, 12 sp carbon atoms), has been subjected to a comprehensive photophysical investigation. At low temperature ( T < 140 K) in a 2-methyltetrahydrofuran (MTHF) glass, the complexes exhibit moderately efficient phosphorescence appearing as a series of narrow (fwhm < 200 cm (-1)) vibronic bands separated by ca. 2100 cm (-1). The emission is assigned to a (3)pi,pi* triplet state that is concentrated on the sp carbon chain, and the vibronic progression arises from coupling of the excitation to the -C[triple bond]C- stretch. The 0-0 energy of the phosphorescence decreases with increasing sp carbon chain length, spanning a range of over 6000 cm (-1) across the series. Transient absorption spectroscopy carried out at ambient temperature confirms that the (3)pi,pi* triplet is produced efficiently, and it displays a strongly allowed triplet-triplet absorption. In the MTHF solvent glass ( T < 140 K), the emission lifetimes increase with emission energy. Analysis of the triplet nonradiative decay rates reveals a quantitative energy gap law correlation. The nonradiative decay rates can be calculated by using parameters recovered from a single-mode Franck-Condon fit of the emission spectra.