Ultrafast spectroscopy of dinaphthylpolyynes

Summary form only given. We investigated experimentally and theoretically the photo-physical properties of a class of linear sp-carbon chains, the α,ω-dinaphthylpolyyne Ar-C2n-Ar, where Ar is the naphthyl group and n is the number of triple bonds. The dynamics of the excited states has been investigated by ultrafast transient absorption spectroscopy and DFT//TDDFT calculations.We found that the role of molecular conformers, in which end-capped naphthalene rings are planar or perpendicular to the polyyne plane, is fundamental for understanding the steady state properties, like the UV-Vis absorption spectra and vibronic transitions, and the ultrafast transient absorption features. Owing to DFT//TD-DFT analysis, we were able to quantify the contributions of each conformer in the UV-Vis absorption spectra (not shown in the figure). Accordingly ultrafast transient absorption measurements revealed the excited states spectral features and dynamics. Figure 1(a) shows the 2D map of the pump-probe signal. The excitation pulse was 150-fs long and centered at 390 nm while the probe pulse was supercontinuum white light from 450 to 750 nm. Figure 1(b) shows two cuts of the 2D map for two different wavelengths. Over the whole spectral bandwidth it is possible to observe a broad photo-induced absorption, which appears instantaneously upon impulsive photo-excitation. A photo-induced absorption signal forms at 600 nm with a time constant of 30 ps and a narrow-band spectral shape.In light of DFT//TD-DFT, these features can be assigned to different transitions. The photo-induced absorption with a fast formation time can be assigned to optical transitions from the excited singlet state to higher molecular singlet levels. The narrow feature centered at 600 nm can be related to an intersystem crossing event bringing to the formation of a triplet state followed by transition between triplet states. In this picture the conformers play a prominent role: we observed a selective pathway of triplet formation, determined by the peculiar conformational structure of dinaphthylpolyyne in the ground state. Understanding the role of end-capped groups opens the way for other possible chemical functionalizations of sp-carbon chain systems in order to govern their chemical stability and allow optical control of their photo-excited properties.