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 pi-donating abilities of X and Y increase systematically in the orders NHMe < -NMe2 < (pyrrolidin-l-yl) and -CN < -Cl < -CCPh, respectively. The results presented include crystal structures of two new 4'-NHMe-T complexes of platinum, as well as absorption, emission, and excited-state Lifetime data for nine complexes. Excited-state lifetimes obtained in deoxygenated dichloromethane vary by a factor of 100, ranging from 24 mu s for [Pt(4'-pyrr-T)CN](+) to 0.24 mu s for [Pt(4'-ma-T)Cl]+, where ma-T denotes 4'(methylamino)-2,2':6',2 ''-terpyridine and pyrr-T denotes 4'-(pyrrolidin-l-y)-2,2':6',2 ''-terpyridine. Analysis of experimental and computational results shows that introducing a simple amine group on the terpyridine and/or a pi-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.