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Described are picosecond and nanosecond time-resolved optical (TRO) spectral and nanosecond time-resolved infrared (TRIR) spectral studies of intermediates generated when the rhodium(I) complexes trans-RhCl(CO)L2 (L = PPh3 (I), P(p-tolyl)3 (II), or PMe3 (III)) are subjected to photoexcitation. Each of these species, which are precursors in the photocatalytic activation of hydrocarbons, undergoes CO labilization to form an intermediate concluded to be the solvated complex RhCl(Sol)L2 (A(i)). The picosecond studies demonstrate that an initial transient is formed promptly (<30 ps), which decays to A(i) with lifetimes ranging from 40 to 560 ps depending upon L and the medium. This is proposed on the basis of ab initio calculations to be a metal-to-ligand charge transfer (MLCT) excited state. Second-order rate constants (kCO) for reaction of the A(i) with CO were determined, and these depend on the nature of L and the solvent, the slowest rate being for A(I) in tetrahydrofuran (kCO = 7.1 x 10(6) M(-1) x s(-1)), the fastest being for A(III) in dichloromethane (1.3 x 10(9) M(-1) x s(-1)). Each A(i) also undergoes competitive unimolecular reaction with solvent to form long-lived transients with TRIR properties suggesting these to be Rh(III) products of oxidative addition. Although this was mostly suppressed by the presence of higher concentrations of CO (which trapped A(i) to re-form the starting complexes in each case), both TRO and TRIR experiments indicate that a fraction of the oxidative addition could not be quenched. Thus, the short-lived MLCT state or a vibrationally hot species formed during the decay of this excited state appears to participate directly in C-H activation.

Syntheses of two analogs of deoxyuridine with N,N-dialkylaniline chromophores are reported. 5-[3-(N-methylphenylamino)propanoyl]-2'-deoxyuridine (1) and 5-[2-(4-N,N-dimethylaminophenyl)ethyl)]-2'-deoxyuridine (2) are prepared by palladium-mediated coupling. Preparation of 2 was facilitated by in situ transient O4-trimethylsilyl protection during alkynylation which suppressed secondary cyclization of the coupling adduct.

Intramolecular emission quenching of a photoexcited ruthenium(II) polypyridine by a covalently linked naphthalene diimide (NDI) has been measured in aqueous buffer both without and with calf thymus DNA. The complex consists of a Ru(2,2'-bipyridine)(2)(2,2'-bipyridine-5-carboxamide)(2+) electron donor covalently attached by way of a -CH(2)CH(2)CH(2)- linker to a 1,4,5,8-naphthalene diimide acceptor (Ru-NDI, 1). The NDI portion of the complex intercalates in calf thymus DNA, as indicated by the hypochromism of its optical absorbance bands and observation of an induced circular dichroism spectrum in the same region. Emission quenching in Ru-NDI has been measured relative to a Ru tris-bpy model lacking the NDI moiety by both lifetime and emission quantum yield techniques. Using lifetime averages, the relative emission quenching is, respectively, 99.1% and 97.9% in aqueous buffer solutions without and with DNA. The emission quenching is ascribed to intramolecular electron transfer within the Ru-NDI complex with an estimated driving force (-DeltaG degrees ) of 0.33 eV. In buffer, the emission decays of Ru-NDI alone are fit well with a triexponential model with lifetimes of 0.34 (0.88), 1.99 (0.11), and 12.6 (0.008) ns (relative amplitude). The emission decays of the DNA-intercalated Ru-NDI complex are also fit well with a triexponential model with lifetimes of 0.31 (0.79), 2.00 (0.13), and 11.8 (0.08) ns. Thus, the fractional amplitudes of the lifetimes change upon DNA intercalation of the complex, while the lifetimes themselves remain essentially the same. The average rates of electron transfer in aqueous buffer without and with DNA are, respectively, 1.6 x 10(9) and 6.8 x 10(8) s(-)(1). The striking result of this study is that the overall character of electron transfer quenching in Ru-NDI is very similar whether or not it is bound to DNA. Intercalation of the NDI in DNA apparently has negligible consequences for electron transfer, implying either that the activation energy and electronic coupling in Ru-NDI are largely unaffected by this, at first glance, seemingly significant environmental change or that changes in these parameters on DNA binding cancel fortuitously.

Comparison of picosecond kinetic and spectroscopic data for Zn octaethylporphine and Fe(III)Cl octaethylporphine with that for Zn-Fe(III)Cl, a cofacial diporphyrin composed of a Zn porphyrin covalently bound to an Fe(III)Cl porphyrin with two chains of five atoms each, supports the assignment of a light-driven electron transfer (k > 10(11)s(-1)) within Zn-Fe(III)Cl to form [Zn(+).-Fe(II)]Cl. The kinetics (k approximately 10(10)s(-1)) and thermodynamics of the reverse electron transfer are compared to those of a similar electron transfer in bacterial photosynthesis, the reduction of an oxidized bacteriochlorophyll dimer, (BChl)(2) (+)., by Fe(II) cytochrome c.