Ultrafast Excited State Dynamics of Pt(II) Chromophores Bearing Multiple Infrared Absorbers

• Published in: Inorganic chemistry Vol. 47; no. 15; pp. 6974 - 6983
• Main Authors: Glik, Elena A, Kinayyigit, Solen, Ronayne, Kate L, Towrie, Michael, Sazanovich, Igor V, Weinstein, Julia A, Castellano, Felix N
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.08.2008
• Subjects: Charge transfer; Chromophores; Esters; Excitation; Infrared; Optical transition; Spectroscopy; Vibration
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic800578h

The paper reports the synthesis, structural characterization, electrochemistry, ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopy associated with two independent d8 square planar Pt(II) diimine chromophores, Pt(dnpebpy)Cl2 (1) and Pt(dnpebpy)(CCnaph)2 (2), where dnpebpy = 4,4′-(CO2CH2- t Bu)2-2,2′-bipyridine and CCnaph = naphthylacetylide. The neopentyl ester substitutions provided markedly improved complex solubility relative to the corresponding ethyl ester which facilitates synthetic elaboration as well as spectroscopic investigations. Following 400 nm pulsed laser excitation in CH2Cl2, the 23 cm−1 red shift in the νCO vibrations in 1 are representative of a complex displaying a lowest charge-transfer-to-diimine (CT) excited state. The decay kinetics in 1 are composed of two time constants assigned to vibrational cooling of the 3CT excited-state concomitant with its decay to the ground state (τ = 2.2 ± 0.4 ps), and to cooling of the formed vibrationally hot ground electronic state (τ = 15.5 ± 4.0 ps); we note that an assignment of the latter to a ligand field state cannot be excluded. Ultrafast TA data quantitatively support these assignments yielding an excited-state lifetime of 2.7 ± 0.4 ps for the 3CT excited-state of 1 and could not detect any longer-lived species. The primary intention of this study was to develop a PtII complex (2) bearing dual infrared spectroscopic tags (CC attached to the metal and CO (ester) attached to the diimine ligand) to independently track the movement of charge density in different segments of the molecule following pulsed light excitation. Femtosecond laser excitation of 2 in CH2Cl2 at 400 nm simultaneously induces a red-shift in both the νCO (−30 cm−1) and the νCC (−61 cm−1) vibrations. The TRIR data in 2 are consistent with a charge transfer assignment, and the significant decrease of the energy of the νCC vibration suggests a considerable contribution from the acetylide ligands in the highest occupied molecular orbital. Therefore, we assign the lowest energy optical transitions in 2 as a combination of metal-to-ligand and ligand-to-ligand charge transfers. The excited-state of 2 is emissive at RT, with an emission maximum at 715 nm, quantum yield of 0.0012, and lifetime of 23 ns.