Excited-State Absorption Properties of Platinum(II) Terpyridyl Acetylides

A comprehensive photophysical study is presented which compares the ground- and excited-state properties of
four platinum(II) terpyridyl acetylide compounds of the general formula [Pt(tBu3tpy)(C⋮CR)]+, where tBu3tpy is 4,4‘,4‘ ‘-tri-tert-butyl-2,2‘:6‘,2‘ ‘-terpyridine and R is an alkyl or aryl group. [Ru(tBu3tpy)3]2+ and the pivotal synthetic precursor
[Pt(tBu3tpy)Cl]+ were also investigated in the current work. The latter two complexes possess short excited-state
lifetimes and were investigated using ultrafast spectrometry while the other four compounds were evaluated using
conventional nanosecond transient-absorption spectroscopy. The original intention of this study was to comprehend
the nature of the impressive excited-state absorptions that emanate from this class of transition-metal chromophores.
Transient-absorbance-difference spectra across the series contain the same salient features, which are modulated
only slightly in wavelength and markedly in intensity as a function of the appended acetylide ligand. More intense
absorption transients are observed in the arylacetylide structures relative to those bearing an alkylacetylide, consistent
with transitions coupled to the π system of the ancillary ligand. Reductive spectroelectrochemical measurements
successfully generated the electronic spectrum of the tBu3tpy radical anion in all six complexes at room temperature.
These measurements confirm that electronic absorptions associated with the tBu3tpy radical anion simply do not
account for the intense optical transitions observed in the excited state of the Pt(II) chromophores. Transient-trapping experiments using the spectroscopically silent reductive quencher DABCO clearly demonstrate the loss of
most transient-absorption features in the acetylide complexes throughout the UV, visible, and near-IR regions
following bimolecular excited-state electron transfer, suggesting that these features are strongly tied to the
photogenerated hole which is delocalized across the Pt center and the ancillary acetylide ligand.