Monodisperse poly(triacetylene) oligomers extending from monomer to hexadecamer: Joint experimental and theoretical investigation of physical properties

A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligome rs ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotection-oxidative Hay oligomerization protocol. The PTA o ligomers exhibit an increasingly deep-yellow color with lengthening of the pi-conjugated backbone, feature excellent solubility in aprotic solvents, a nd exhibit melting points up to > 220 degrees C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to invest igate the evolution of the physico-chemical effects in PTAs beyond the line ar 1/n versus property regime into the higher oligomer region where saturat ion becomes apparent. We report the results of joint experimental and theor etical studies, including analysis of the C-13 NMR spectra, evaluation of t he linear (UV/Vis) and nonlinear [third-harmonic generation (THG) and degen erate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the he xadecameric rod, an increasingly facile one-electron reduction step is obse rved, showing at the stage of the dodecamer, a leveling off tendency from t he linear correlation between the inverse number of monomer units and the f irst reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a,pi-electron-delocalization leng th of about n = 10 monomeric units, which corroborates well with the oligom eric length for which in the C-13 NMR spectrum C(sp(2)) and C(sp) resonance s start to overlap. Third-harmonic generation (THG) and degenerate four-wav e mixing (DFWM) measurements revealed for the second-order hyperpolarizabil ity gamma a power law increase gamma . alpha . n(a) for oligomers up to the octamer with exponential factors a = 2.46 +/- 0.10 and a = 2.64 +/- 0.20, respectively, followed by a smooth saturation around n = 10 repeating units . The power law coefficient tr calculated with the help of the valence effe ctive Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalis m corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap )-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units d isplaying a strong electron-hole symmetry. The on-set of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.