Long-lived emissions from 4'-substituted Pt(trpy)Cl+ complexes bearing aryl groups. Influence of orbital parentage

Pt(trpy)Cl+, where trpy denotes 2,2':6',2' '-terpyridine, is a versatile binding agent but has a limited photochemistry due to a short excited-state lifetime. However, this work shows that the introduction of aryl substituents at the 4' position of the trpy ligand drastically alters the picture. For the substituents phenyl, p-methoxyphenyl, 1-naphthyl, 2-naphthyl, 9-phenanthrenyl, and 1-pyrenyl, the ligand abbrevations are 4'-Ph-T, 4'-pMeOPh-T, 4'-Npl-T, 4'-Np2-T, 4'-Phe9-T, and 4'-Pyre1-T, respectively. Techniques utilized include electrochemistry as well as absorption and emission spectroscopies. While the lowest energy excited states of Pt(4'-Ph-T)Cl+ and the parent complex Pt(trpy)Cl+ exhibit mainly metal-to-ligand charge-transfer (MLCT) character, the emitting state takes on aryl-to-trpy intraligand charge-transfer (ILCT) character as the substituents become more electron-donating. Studies of Zn(trpy)Cl2, its aryl-substituted analogues, and the free ligands themselves provide information about the relative energies of participating ILCT and intraligand 3pi-pi excited states. Even though the emission energy decreases when larger aryl groups are present, the emission lifetime increases all the way from 85 ns for Pt(4'-Ph-T)Cl+ to 64 micros for Pt(4'-Pyre1-T)Cl+. (Data from deoxygenated, room-temperature dichloromethane solution.) Intraligand character appears to dominate in the case of Pt(4'-Pyre1-T)Cl+, which is unique in the series in that it exhibits singlet and triplet emissions in solution. In aerated solution the complex shows prompt as well as delayed fluorescence. Finally, studies in donor media establish that the introduction of intraligand character inhibits solvent-induced exciplex quenching.