Rational synthetic design of well-defined Pt(bisethynyl)/Zn(porphyrin) oligomers for potential applications in photonics

Well-defined oligomers of 1, 2, 3 and 4 units built upon the very soluble bis-1,15-(1,4-ethynylbenzene)-3,7,13,17-tetramethyl-2,8,12,18-tetrakis(n-hexyl) zinc(II) porphyrin ligand and the trans-bis(tri-n-butylphosphine)platinum(II) linker, with acetylene or trimethylsilane as end groups, has been prepared in the presence of a dichloromethane/diethylamine mixture (1 : 1 v/v) and CuX (X = Cl, I) at room temperature, analogue to a Sonogashira coupling. The new monodisperse organometallic oligomers were characterized by 1H, 31P NMR, UV-visible spectroscopies and MALDI-TOF mass spectrometry. The methyl groups placed at the 3,7,13,17-positions induces the locking of the C6H4 fragment in a perpendicular conformation with respect to the zinc(II) porphyrin chromophore, hence removing conjugation as corroborated by the almost total absence of spectral shift of the Soret and Q-bands upon increasing the number of units. Despite this feature, exciton coupling in the Soret band is noted at both room temperature and 77 K. The photophysical parameters, fluorescence lifetimes and quantum yields are practically constant going from the monomer, dimer and tetramer, and as a function of the monitored fluorescence wavelength, all indicating that the excitonic behavior (excitation energy delocalization) is minimal, which is consistent with the weak exciton coupling constants and the lack of conjugation of the π-system. The synthetic methodology can provide longer well-defined oligomers as the presented products were still very soluble even when the number of unit was 4.