π Donation and Its Effects on the Excited-State Lifetimes of Luminescent Platinum(II) Terpyridine Complexes in Solution

Introducing electron-donating groups extends the excited-state lifetimes of platinum­(II)–terpyridine complexes in fluid solution. Such systems are of interest for a variety of applications, viz., as DNA-binding agents or as components in luminescence-based devices, especially sensors. The complexes investigated here are of the form [Pt­(4′-X-T)­Y]+, where 4′-X-T denotes a 4′-substituted 2,2′:6′,2″-terpyridine ligand and Y denotes the coligand. The π-donating abilities of −X and −Y increase systematically in the orders −NHMe 2 + to 0.24 μs for [Pt­(4′-ma-T)­Cl]+, where ma-T denotes 4′-(methylamino)-2,2′:6′,2″-terpyridine and pyrr-T denotes 4′-(pyrrolidin-1-yl)-2,2′:6′,2″-terpyridine. Analysis of experimental and computational results shows that introducing a simple amine group on the terpyridine and/or a π-donating coligand engenders the emitting state with intraligand charge-transfer (ILCT) and/or ligand–ligand charge-transfer (LLCT) character. The excited-state lifetime increases when the change in orbital parentage lowers the emission energy, suppresses quenching via d–d states, and encourages delocalization of the excitation onto the ligand(s). At some point, however, the energy is low enough that direct vibronic coupling to the ground-state surface becomes important, and the lifetime begins to decrease again.