Syntheses, Characterization, and Photophysical Properties of Conjugated Organometallic Pt-Acetylide/Zn(II) Porphyrin-Containing Oligomers

Two conjugated organometallic oligomers of the type (−CCPt(L)2CC(ZnP)−)n and model compounds [PhCCPt(L)2CC(ZnP)CCPt(L)2CCPh] with L = tri(n-butyl)-phosphine and ZnP = zinc(II)(10,20-bis(Ar)porphyrine) (Ar = mesityl (Mes; P1 and M1) or 3,4,5-trihexadecyloxyphenyl (P2 and M2)) were synthesized and characterized (1H and 31P NMR, HRMS, elemental analysis, IR, GPC, and TGA). GPC indicates that P1 and P2 exhibit respectively ∼6 and ∼3 units with a polydispersity of 1.4. M1 was also characterized by X-ray crystallography. The Pt···Pt separation in M1 is 1.61 nm, which makes P1 ∼ 9.6 nm long. The spectral measurements show that the absorption and photoluminescence spectra of M1, M2, P1, and P2 are remarkably red-shifted. For example, the low energy Q-band is observed at 677 ± 1 nm in comparison with their precursors HCC(ZnP)CCH, L1 and L2 (Ar = mesityl (Mes; L1) or 3,4,5-trihexadecyloxyphenyl (L2)), both at 298 and 77 K, for which the Q-band is observed at 622 nm. The photophysical parameters, fluorescence lifetimes (τF), and quantum yields (ΦF) show a slight decrease by a factor of ∼2 (at most 3) following the trend L1 ∼ L2 > M1 ∼ M2 > P1 ∼ P2, a trend explained by a combination of the heavy atom effect and an increase in internal conversion rate due to the increase in oligomer dimension. This small variation of the photophysical data for materials of a few nm in dimension contrasts with the larger change in τP, phosphorescence lifetimes of the Pt-containing unit in the (−C6H4CCPt(L)2CC−CC6H4(ZnP)−)n oligomers with n = 3, 6, and 9 reported earlier (Liu, L.; Fortin, D.; Harvey, P. D. Inorg. Chem. 2009, 48, 5891−5900). In this later case, τP decreased by steps of an order of magnitude as n increased from 3 to 6 to 9. This decrease was explained by a T1 energy transfer from the Pt unit (donor) to MP (acceptor) in combination with an excitonic process (energy delocalization). Because of the full conjugation in P1 and P2, these oligomers behave as distinct molecules, and no energy transfer occurs. These properties make these materials suitable candidates for photocell applications.