This Research award in the Inorganic, Bioinorganic and Organometallic Chemistry program supports fundamental studies by Professor David McMillin at Purdue University on the development of new reporter probes that contain a transition metal ion and emit light after electronic excitation. The investigations involve different metal ions, including copper, ruthenium, and platinum, each of which is incorporated into a specifically designed coordination sphere. The focus is mainly on systems that offer open coordination sites where interactions occur in the excited state and provide coupling to the local environment. The systematic approach provides for the development of new, sterically accommodating systems with programmable binding properties. From the educational perspective, the Chemistry Department at Purdue University and the McMillin group have a strong track record of training students from underrepresented groups. In terms of chemistry, the new luminophores that result are applicable to many areas including supramolecular chemistry, molecular sensing, and molecular devices that utilize solar energy.
Background. Photochemistry can make important contributions to the solutions of many topical problems and forms a basis for many applications, including light-activated sensors and probes, while photodynamic therapy represents a minimally invasive approach to treatment. To optimize the desired photochemistry for any application, one needs to map the energy levels and the decay pathways that determine the lifetime of the active photoexcited state. This research developed new luminescent complexes incorporating a copper, ruthenium, or platinum metal center. Cationic porphyrins. One set of studies focused on binding interactions of cationic porphyrins with double-stranded RNA as a function of the base composition of the host as well as the steric demands of the porphyrin ligand. Previous workers have generally utilized B-form DNA as host, because RNA is a globular structure. However, in places the chain can fold back on itself and give rise to stem loop, or hairpin, structures. Accordingly, double-stranded RNA structures can be useful as recognition elements for antiviral response and for targeting RNA viruses like HIV. This work employs hairpin-forming sequences as programmable sources of double-stranded RNA. Work during the project period focused on new sterically friendly, copper-containing porphyrins with a pair of cis (adjacent to each other) or trans (opposite each other) substituents. With double-stranded domains of RNA as hosts, the strong hypochromic responses and emissive properties of copper-containing forms establish that intercalation is the preferred binding motif, not only for sterically friendly dicationic porphyrins, but the bulky commercial tetracationic analogue as well. However, bound forms of compact porphyrins are invariably less prone to axial attack on the time scale of the excited-state lifetime. Intriguing differences in the induced circular dichroism signals are worth noting. The signals are opposite in sign for the trans and tetracationic porphyrins even though both apparently bind by intercalating into the RNA host. The exquisite sensitivity may be due to the fact that the induced response depends on the base arrangement within the host as well as ligand placement relative to the base pairs. Mixed-ligand luminophors containing ruthenium(II). The exploration of the photochemical and photophysical properties of polypyridine complexes of ruthenium(II) has a long history. However, many systems have very short excited-state lifetimes in fluid solution due to thermally assisted deactivation via metal-centered states. Work focused on mixed ligand systems with a terpyridine ligand (X-NNN) bearing an electron-donating substituent. The co-ligands are a bipyridine with a high electron affinity and a cyanide to complete the coordination sphere. Varying the electron-donating group and/or the solvent is an effective way to tune the energy and lifetime of the emitting state which extends as high as 175 ns. That represents a twenty-fold improvement in lifetime relative to the original prototype. A simple theoretical model proves capable of rationalizing all the experimental lifetimes. It suggests that increasing the donor ability of the substituent lowers the energy of the emitting state and reduces the possibility of thermally activated decay. However, direct non-radiative decay to the ground state begins to reduce the excited-state lifetime whenever the emission maximum shifts beyond 750 nm. Within those limits there is inevitably a maximum attainable lifetime, regardless of the method of tuning. Versatile platinum(II) terpyridines. Platinum(II) differs from ruthenium(II) in having two more d electrons and is prone to forming complexes with two open coordination sites. Thus, a typical mixed-ligand complex contains a tridentate terpyridine ligand and a complementary monodentate anionic ligand. A tridentate CNN ligand fused with phenazine complexes to platinum to give a new complex that is highly emissive in non-coordinating solvents. The luminescent excited state is intriguing because it is subject to novel, regiospecific quenching. The 'hole' induced by photoexcitation spends part of the time at platinum, while the electron counterpart spends the bulk of its time on the phenazine moiety. One of the upshots is that Lewis bases react with the excited state and induce emission quenching by attacking the metal center. On the other hand, the formally anionic phenazine moiety is subject to attack by hydrogen-bonding Lewis acids at its nitrogen centers. Cyanoacetic acid exhibits a relatively high quenching rate constant, which decreases 30% when the acid is in the (NC)CH2CO2D form. That suggests the quenching occurs by hydrogen bond formation as opposed to net proton transfer. Other studies focused on complexes of the substituted terpyridine X-NNN, along with a complementary ligand denoted Y. Varying the electron-donating abilities of the -X and -Y groups provided a total of nine complexes. Remarkably, the associated excited-state lifetimes differed by a factor of 100 over the series in de-oxygenated dichloromethane. As in the ruthenium complexes discussed above, the excited-state lifetime increases when the change in orbital parentage lowers the emission energy, suppresses quenching via metal-centered states, and encourages delocalization of the excitation onto the ligand.
