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Excited-State Absorption of 4′-(5”’-R-pyrimidyl)-2, 2′:6′, 2”-terpyridyl Platinum(II) Phenylacetylide Complexes
Wenfang Sun,Yunjing Li, Timothy M. Pritchett, Zhiqiang Ji and Joy E. Haley

The singlet excited-state absorption properties of five 4′-(5”’-Rpyrimidyl)-2, 2′:6′, 2″-terpyridyl platinum(II) phenylacetylide complexes were investigated using fs pump-probe UV-Vis spectroscopy. These complexes exhibit broad, moderately intense S1Sn absorption from 500 to 800 nm, which could be attributed to the 1MLCT state. The S1Sn absorption band energy is influenced by the substituent on the 5”’-position of the pyrimidyl ring, with an electron-donating substituent increasing the energy and electron-withdrawing substituent decreasing it. The singlet excited-state lifetime deduced from the decay of the fs transient absorption is in the range of 37–139 ps. Open-aperture Z-scan experiments using ns and ps lasers at 532 nm were carried out for all five complexes 1–5 in CH3CN solution, and the wavelength dispersion characteristics of 1 were investigated using ps laser pulses at various wavelengths. Reverse saturable absorption (RSA) is observed in all complexes at 532 nm at both ns and ps pulse widths, and RSA occurs in 1 over the range 500–600 nm at ps pulse widths. By fitting the experimental Z-scan data using a five-level dynamic model, one obtains values for the singlet and triplet excited-state absorption cross-sections (σs and σT, respectively); for 1–5, these vary from (18 ± 1) × 10−18 cm2 to (50 ± 5) * 10−18 cm2 for σs and (11 ± 1) * 10−18 cm2 to (14 ± 2) * 10−18 cm2 for σT at 532 nm, corresponding to ratios σs/σg of 6.5–29.8 and ratios σT/σg of 2.8–11.2. Complexes 1, 2 and 4 exhibit much larger ratios σs/σg than those of 3 and 5. Moreover, in complex 1 the σs/σg ratio shows a sharp rise from 1.9 at 500 nm to 260 at 600 nm.

Keywords: Excited-state absorption, ground-state absorption, cross section, transient difference absorption, Z scan, platinum complex.

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