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
< −NMe2 < −(pyrrolidin-1-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 μs
for [Pt­(4′-pyrr-T)­CN]+ 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.